Re: [PATCH v2 4/7] ARM: davinci: Add support for an L3RAM gen_pool

[Sekhar Nori]

On 10/1/2012 7:20 PM, Ben Gardiner wrote:
> On Mon, Oct 1, 2012 at 8:32 AM, Matt Porter <mporter@xxxxxx> wrote:
>> On Mon, Oct 01, 2012 at 05:34:02PM +0530, Sekhar Nori wrote:
>>> Hi Matt,
>>>
>>> On 9/29/2012 1:07 AM, Matt Porter wrote:
>>>> L3RAM (shared SRAM) is needed for use by several drivers.
>>>> This creates a genalloc pool and a hook for the platform code
>>>> to provide the struct gen_pool * in platform data.
>>>>
>>>> Signed-off-by: Matt Porter <mporter@xxxxxx>
>>>
>>> I am not sure if any of the DaVinci devices have a need to allocate from
>>> *both* ARM RAM and shared RAM. Shared RAM is not present on all DaVinci
>>> devices AFAIR, and on DA850, there is just 8KB ARM RAM so I am not sure
>>> if there is much point in trying to allocate from there.
>>>
>>> Can you instead see if Ben's earlier patch[1] to use shared RAM for SRAM
>>> allocation on DA850 makes sense for your case? If yes, can you repost
>>> with Ben's patch included in your series instead of this patch? I would
>>> prefer that over creating a new pool for shared RAM.
>>
>> Hrm, I did look at Ben's earlier patch. The reason I added a separate
>> pool mostly was so I didn't have to touch the PM code at all. That can
>> continue using the private SRAM API with the ARM RAM as it is now. The
>> idea here was to allow that to be separate since no other bus masters
>> can access the ARM RAM anyway and do something that didn't require
>> regression testing PM. Also, I figured there's really no reason to use
>> even a tiny bit of the shared SRAM on PM if we have that ARM RAM there
>> and working fine for that use case.
>> [...]
> 
> I agree with Matt. Preserving the use of the ARM RAM (8K on L138 -- as
> you said, Sekhar) in any fashion is preferable to moving suspend
> support into shared RAM. There is more of it (128K on L138) but also
> more pressure on allocations there since there are more clients.

There is where I would like to see more information on who the potential
clients are. Even if DSP takes away 64K of the shared RAM on OMAP-L138,
there should be more than enough for PM, Audio and PRU. I haven't
checked the PM code size lately but it should be fairly small and I can
check the actual number if that helps. So, adding a new pool just to
save on those bytes doesn't sound like helping a lot.

> I appreciate that you are trying to preserve prior efforts in
> attempted merging of SRAM support -- thank you for that; however, that
> patch [1] was just an import of Subashish Ghosh's patch [2]  -- I
> chose _that_ implementation option then mainly because I imagined it
> would be the least risky to get accepted upstream and not because of
> any particular technical merits.

Its not a question of prior effort since Matt has already put in the
effort too. I am yet unconvinced that we need to add support to manage
two blocks of SoC internal RAM on DA850 in the kernel today. That's all.

Thanks,
Sekhar
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