pxa_camera: DMA alignment requirement

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 ?

--
Robert

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Quoted 5 lines · 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 ?

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

From what I read on the PXA manual, the only constraint is 16bytes

Quoted 6 lines · Mike Rapoport

Quoted 5 lines · 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 ?

Quoted 2 lines

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

Ah, so that's it, thanks.

Quoted 1 line

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.

--
Robert

Quoted 3 lines

Quoted 1 line

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.

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