mt76x0u: Probing issues on Raspberry Pi 3 B+

Quoted 4 lines · Stanislaw Gruszka

Quoted 2 lines · Stefan Wahren

this is a misunderstanding. The warning is about memory alignment to 32 bit addresses, not about page alignment. This is a typical ARM restriction. Maybe we need to make sure in mt76 that the DMA buffer needs to be aligned. But it's also possible that the warning isn't the root cause of our problem.

Quoted 11 lines

I see, it needs 4 bytes alignment . There is already dwc2 code checks that and allocate new buffer if the alignment is not right:
dwc2_alloc_dma_aligned_buffer(), but it does nothing if urb->sg is not NULL. I thought mt76usb already provide aligned buffers, but looks it does not for one TX special case, which are PROBE REQUEST frames. Other frames are aligned by inserting L2 header pad. One solution for this would be just submit urb with NULL sg (same as Lorenzo's patches do, but still allocating buffers via buf->sg), but I think, you have right, we should provide 4 bytes aligned buffers by default as other DMA hardware may require that. I'm attaching yet another patch to test, which fix up alignment for PROBE REQUEST frames.

This approach looks completely wrong to me. MMIO based hardware does not need 4-byte aligned buffers at all, other USB controllers do not need this either.
As Lorenzo already pointed out, re-aligning the buffer is *very* expensive, so we should not do this in the driver just to work around quirks in a particular USB host driver.

The way I see it, we have two choices.
1. Fix dwc2 to do its alignment quirk for the urb->sg != NULL case
2. Rely on urb->transfer_buffer and keep urb->sg NULL