[maemo-developers] ARM MMU questions

[From: Simon Pickering]

> From the pdf it looks like I should be able to map a section of 1
> Megabytes and that should use a single TLB on the DSP (and we 
> know there
> are ~6 TLBs available). There's a gap in the N800's memory 
> space (in the
> same location as the 770's FRAMEBUFFER section) which is 
> 0x100000 bytes
> long (whether these are 8- or 16-bit bytes is anyone's 
> guess). I'll try
> mapping this section and report what happens.

Right, as long as one chooses a name other than FRAMEBUFFER for the
memory section (this name has special meaning and is treated differently
by dsp_dld and the kernel code), it's possible to map a region of memory
that is 0x100000 bytes (8-bit) long (this is as long a region as I've
tried. Longer may be possible, I must test this out). I've no idea how
many TLBs this takes as I'm not told in the dmesg output (but I presume
this should take a single TLB).

I should note here that when declaring the array sizes in the DSP code,
one needs to use 16-bit chars while everything on the ARM side (which
obviously includes the arrays in the ARM-side code, but also includes
the memory section sizes in the /lib/dsp/dsp_dld_avs.conf file) should
be in terms of 8-bit chars.

The large memory sections in the dsp_dld_avs.conf file are 0x100000
8-bit bytes long. Therefore, on the DSP-side you need to declare an
array of size 0x80000 (16-bit) chars.

The following is the command file and output I used to test this:

Using a command file containing the following:


MEMORY {
   PAGE 0: FRAMEBUFFER1:  origin = 0x300000, len = 0x100000
}

SECTIONS {
        sharedmem_mmap_buffer > FRAMEBUFFER1 
}


(note it's FRAMEBUFFER1 not FRAMEBUFFER I'm using, and note that this is
a handy gap in the pre-existing memory map on the N800. On the 770 this
gap is filled by a section named FRAMEBUFFER...).

and when running the test_sharedmem code one obtains the following:

>From dsp_dld:

device sharedmem is requesting for TADD.
starting TADD process for device sharedmem.
loading /lib/dsp/modules/sharedmem.o.
mapping external memory: adr = 0x300000, size = 0x100000
adding _task_sharedmem(@145120) into system.
dsp_dld: event detected.
device sharedmem is requesting for TDEL.
starting TDEL process for device sharedmem.
cleaning task sharedmem...
releasing external memory map: adr = 0x300000
unloading /lib/dsp/modules/sharedmem.o.

>From dmesg:

[52735.006896] MMU: mapping in ARM MMU, v=0xe0300000, p=0x86700000,
sz=0x100000
[52735.084777] mbox: sending   seq=0, cmd=62:00(TADD:task 0), data=0000
[52735.085662] mbox: receiving seq=1, cmd=62:05(TADD:task 5), data=0000
[52735.085754] mbox: sending   seq=0, cmd=60:05(TCFG:task 5), data=0000
[52735.085845] mbox: receiving seq=0, cmd=54:01(KFUNC:PWR DSP /DOWN),
data=0004
[52735.085968] mbox: receiving seq=1, cmd=60:05(TCFG:task 5), data=0000
[52735.086029] omapdsp: task 5: name sharedmem
[52735.086273] mbox: sending   seq=0, cmd=30:05(TCTL:task 5), data=0000
[52735.086303] omapdsp: taskdev sharedmem enabled(tid:5).
[52735.086364] mbox: receiving seq=0, cmd=54:01(KFUNC:PWR DSP /DOWN),
data=0004
[52735.086822] mbox: receiving seq=1, cmd=54:01(KFUNC:PWR DSP /DOWN),
data=0004
[52735.088958] omapdsp: mmap info: vmadr = 4000c000, padr = 86700000,
len = 100000
[52735.089660] mbox: sending   seq=0, cmd=10:05(WDSND:task 5), data=0400
[52735.090179] mbox: receiving seq=0, cmd=10:05(WDSND:task 5), data=040a
[52735.090332] mbox: receiving seq=1, cmd=54:01(KFUNC:PWR DSP /DOWN),
data=0004
[52735.106048] mbox: sending   seq=0, cmd=63:05(TDEL:task 5), data=0000
[52735.106353] mbox: receiving seq=0, cmd=63:05(TDEL:task 5), data=0000
[52735.107238] mbox: receiving seq=1, cmd=54:01(KFUNC:PWR DSP /DOWN),
data=0004
[52735.108306] omapdsp: taskdev sharedmem disabled.
[52735.114044] MMU: unmapping in ARM MMU, v=0xe0300000, sz=0x100000
[52735.114257] MMU: freeing 0x100000 bytes @ adr 0xc6700000

So all works well in this case.

If I try to name my memory section FRAMEBUFFER (same code, just changing
the name in the command file), I'm back to the original problematic sort
of output:

dsp_dld output:

dsp_dld: event detected.
dsp_dld: event detected.
device sharedmem is requesting for TADD.
starting TADD process for device sharedmem.
loading /lib/dsp/modules/sharedmem.o.
exporting framebuffer to 0x300000
FBEXPORT failed
adding _task_sharedmem(@145120) into system.

dmesg output:

[52209.555297] omapdsp: 3 frame buffers found. we use first one.
[52209.555389] MMU: mapping in ARM MMU, v=0xe0300000, p=0x87e00000,
sz=0x10000
[52209.555480] MMU: mapping in ARM MMU, v=0xe0310000, p=0x87e10000,
sz=0x10000
[52209.555541] MMU: mapping in ARM MMU, v=0xe0320000, p=0x87e20000,
sz=0x10000
[52209.555603] MMU: mapping in ARM MMU, v=0xe0330000, p=0x87e30000,
sz=0x10000
[52209.555694] MMU: mapping in ARM MMU, v=0xe0340000, p=0x87e40000,
sz=0x10000
[52209.555755] MMU: mapping in ARM MMU, v=0xe0350000, p=0x87e50000,
sz=0x10000
[52209.555816] MMU: TLB is full.
[52209.555847] MMU: unmapping in ARM MMU, v=0xe0350000, sz=0x10000
[52209.555938] MMU: unmapping in ARM MMU, v=0xe0300000, sz=0x10000
[52209.555999] MMU: unmapping in ARM MMU, v=0xe0310000, sz=0x10000
[52209.556060] MMU: unmapping in ARM MMU, v=0xe0320000, sz=0x10000
[52209.556152] MMU: unmapping in ARM MMU, v=0xe0330000, sz=0x10000
[52209.556213] MMU: unmapping in ARM MMU, v=0xe0340000, sz=0x10000
[52209.556243] omapdsp: exmap failure.
[52209.603820] MMU: DSP address 00300000 / size 00100000 is not valid!
[52209.618957] mbox: sending   seq=0, cmd=62:00(TADD:task 0), data=0000
[52209.619781] mbox: receiving seq=1, cmd=62:05(TADD:task 5), data=0000
[52209.619873] mbox: receiving seq=0, cmd=54:01(KFUNC:PWR DSP /DOWN),
data=0004
[52209.637329] mbox: sending   seq=0, cmd=60:05(TCFG:task 5), data=0000
[52209.637756] mbox: receiving seq=1, cmd=60:05(TCFG:task 5), data=0000
[52209.638427] mbox: receiving seq=0, cmd=54:01(KFUNC:PWR DSP /DOWN),
data=0004
[52209.639770] omapdsp: task 5: name sharedmem
[52209.639862] omapdsp: illegal mmap buffer address(0xe0300000) or
length(0x100000).
[52209.639892]   It needs to be page-aligned and located at external
memory.
[52209.639923] omapdsp: deleting the task...
[52209.640075] mbox: sending   seq=0, cmd=63:05(TDEL:task 5), data=0001
[52209.640380] mbox: receiving seq=1, cmd=63:05(TDEL:task 5), data=0000
[52209.640533] mbox: receiving seq=0, cmd=54:01(KFUNC:PWR DSP /DOWN),
data=0004
[52209.641418] omapdsp: task attach failed for sharedmem!

I'll investigate this further (i.e. more trawling through the kernel
code to work out why the sharing of this region is handled differently),
but at least this proves that large memory regions can be shared without
running out of TLBs. 

Another thing I need to look at is whether I can share some large shared
memory regions on the 770 at higher addresses in the DSP memory map (to
be used by mplayer to avoid memcpy operations on frames). The
interesting points here are whether there's enough room in the shareable
region (iirc there should be 16Mb or so in total, so this should be ok)
and whether I can share a region which is more than 1 Megabyte (8-bit)
in size. The Nokia configuration only uses memory sections that are up
to 0x100000 (8-bit) bytes in size, so I wonder if this is a limit. 

Cheers,


Simon