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

[Ben Gardiner]

On Tue, Oct 2, 2012 at 7:13 AM, Sekhar Nori <nsekhar@xxxxxx> wrote:
> 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.

Good points. I suppose that the list would decrease also on L138 if
davinci-pcm ping-pong buffers were removed from SRAM.

>> 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.

Understood. Thank you, Sekhar.

Best Regards,
Ben Gardiner

---
Nanometrics Inc.
http://www.nanometrics.ca
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