mt76x0u: Probing issues on Raspberry Pi 3 B+

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Quoted 42 lines · Lorenzo Bianconi

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Quoted 32 lines · Felix Fietkau

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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

I agree, if this is only needed for dwc2. Though I would investigate if this is not a bug on other platforms as well.
From what I can see, using Lorenzo's patches seems to be the better

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solution, since they avoid these corner cases in dwc2 (and maybe other drivers as well). I will apply them and then we'll see if we need to do any further improvements later on.

They work on rpi dwc2, but they do not address root of the problem. There is clearly something wrong how mt76usb handle SG, what is not fixed. And adding disable_usb_sg module parameter for hcd's supporting SG should be red flag.

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I think we're simply dealing with multiple issues here, only some of which are fixed by Lorenzo's patches.
I'm pretty sure it's still wrong for mt76 to try to align its buffers, since the Linux USB API supports non-aligned transfer buffers and it should be up to the controller driver to deal with that.

Agree.

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dwc2 tries to do that, but that has limitations which I already pointed out and which are properly dealt with by Lorenzo's patches.

I planed to just accept current solution, but I started to work on patch that remove len, sglen arguments from mt76u_buf_alloc() and use q->buf_size and SKB_WITH_OVERHEAD(q->buf_size) directly and realized how related code is now tangled.

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Would be ok to send this patch with proper changelog as fix for RPI against wireless-drivers and cc:stable (assuming it works and really fix things on RPI) and revert Lorenzo's patches in -next ?

Hi Stanislaw,

what is the advantage of doing so?

To have small fix proper for -stable to fix the problem in 4.20 and 4.19, and have simpler code.

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merging the series I sent we will have a pretty simple approach, just a single routine that allocates the rx buffers in the control path according to the operating mode.

So will you do the backport of your patches and post them to -stable ? Do you think greg-kh will accept those ?

Stanislaw