Re: pxa_camera: DMA alignment requirement

[Mike Rapoport <mike@compulab.co.il>]

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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