pxa_camera: DMA alignment requirement

pxa_camera: DMA alignment requirement

[Robert Jarzmik]

Hello,

In the pxa camera driver, the 3 DMA channels used are MMU page aligned. Does
somebody remember where that constraint comes from ?

I'm wondering because the planar YUV format generated is something like this for
a 640 * 480 image :
 - Y plane => 640 * 480 bytes = 307200 (and this happens to be a multiple of
 pagesize : 307200 = 4096 * 75)
 - U plane => 640 * 480 / 2 bytes = 153600 (and this is not a multiple of 4096)
 - padding to reach next page : 2048 bytes
 - V plane => 153600 bytes
 - padding to reach next page : 2048 bytes

This means a user space library should be kernel pagesize aware to transform the
output image. I don't really understand the necessity of page aligned DMA
channels. Would someone tell me please ?

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Robert

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Re: pxa_camera: DMA alignment requirement

[Mike Rapoport]

Robert Jarzmik wrote:
> Hello,
> 
> In the pxa camera driver, the 3 DMA channels used are MMU page aligned. Does
> somebody remember where that constraint comes from ?
> 
> I'm wondering because the planar YUV format generated is something like this for
> a 640 * 480 image :
>  - Y plane => 640 * 480 bytes = 307200 (and this happens to be a multiple of
>  pagesize : 307200 = 4096 * 75)
>  - U plane => 640 * 480 / 2 bytes = 153600 (and this is not a multiple of 4096)
>  - padding to reach next page : 2048 bytes
>  - V plane => 153600 bytes
>  - padding to reach next page : 2048 bytes
> 
> This means a user space library should be kernel pagesize aware to transform the
> output image. I don't really understand the necessity of page aligned DMA
> channels. Would someone tell me please ?

As far as I remember, the buffers should be page aligned to allow overlaying of
captured data.

> --
> Robert
> 
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Sincerely yours,
Mike.

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Re: pxa_camera: DMA alignment requirement

[Robert Jarzmik]

Mike Rapoport <mike@compulab.co.il> writes:

> Robert Jarzmik wrote:
>> Hello,
>> 
>> In the pxa camera driver, the 3 DMA channels used are MMU page aligned. Does
>> somebody remember where that constraint comes from ?
>> 
>
> As far as I remember, the buffers should be page aligned to allow overlaying of
> captured data.
Ah, so that's it, thanks.

>From what I read on the PXA manual, the only constraint is 16bytes
alignement. Would you also remember who wrote the code, so that I can ask him
where he read about page alignement constraint please ?

Cheers.

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Robert

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Re: pxa_camera: DMA alignment requirement

[Mike Rapoport]

Robert Jarzmik wrote:
> Mike Rapoport <mike@compulab.co.il> writes:
> 
>> Robert Jarzmik wrote:
>>> Hello,
>>>
>>> In the pxa camera driver, the 3 DMA channels used are MMU page aligned. Does
>>> somebody remember where that constraint comes from ?
>>>
>> As far as I remember, the buffers should be page aligned to allow overlaying of
>> captured data.
> Ah, so that's it, thanks.
> 
>>From what I read on the PXA manual, the only constraint is 16bytes
> alignement. Would you also remember who wrote the code, so that I can ask him
> where he read about page alignement constraint please ?

Just by coincidence it was me :)
I used pxafb_overlay from [1], and my (mis)understanding of what it does added
the page alignment constrain. Now looking at the overlay code once more, I'm not
sure if it's necessary.


---
[1] http://www.rpsys.net/openzaurus/patches/archive/pxa27x_overlay-r8.patch

> Cheers.
> 
> --
> Robert
> 

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Sincerely yours,
Mike.

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Re: pxa_camera: DMA alignment requirement

[Robert Jarzmik]

Mike Rapoport <mike@compulab.co.il> writes:

>>>From what I read on the PXA manual, the only constraint is 16bytes
>> alignement. Would you also remember who wrote the code, so that I can ask him
>> where he read about page alignement constraint please ?
>
> Just by coincidence it was me :)
> I used pxafb_overlay from [1], and my (mis)understanding of what it does added
> the page alignment constrain. Now looking at the overlay code once more, I'm not
> sure if it's necessary.
>
> ---
> [1] http://www.rpsys.net/openzaurus/patches/archive/pxa27x_overlay-r8.patch

Just for the record, I made a try or two, changing the alignment from page (4096
bytes) into 16 bytes (PAGE_ALIGN -> ALIGN(x,16)). And surprise, I get a lot of
DMA bus errors ...

I'm still investigating, there's something weird in there ...

Cheers.

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Robert

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