linux-kernel - RE: [PATCH v3] media: uvc: don't do DMA on stack

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Date:   Tue, 22 Jun 2021 14:21:27 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Alan Stern' <stern@...land.harvard.edu>
CC:     'Mauro Carvalho Chehab' <mchehab+huawei@...nel.org>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        "linuxarm@...wei.com" <linuxarm@...wei.com>,
        "mauro.chehab@...wei.com" <mauro.chehab@...wei.com>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-media@...r.kernel.org" <linux-media@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: RE: [PATCH v3] media: uvc: don't do DMA on stack

From: Alan Stern
> Sent: 22 June 2021 14:29
...
> > Thought...
> >
> > Is kmalloc(1, GFP_KERNEL) guaranteed to return a pointer into
> > a cache line that will not be accessed by any other code?
> > (This is slightly weaker than requiring a cache-line aligned
> > pointer - but very similar.)
> 
> As I understand it, on architectures that do not have cache-coherent
> I/O, kmalloc is guaranteed to return a buffer that is
> cacheline-aligned and whose length is a multiple of the cacheline
> size.
> 
> Now, whether that buffer ends up being accessed by any other code
> depends on what your driver does with the pointer it gets from
> kmalloc.  :-)

Thanks for the clarification.

Most of the small allocates in the usb stack are for transmits
where it is only necessary to ensure a cache write-back.

I know there has been some confusion because one of the
allocators can add a small header to every allocation.
This can lead to unexpectedly inadequately aligned pointers.
If it is updated when the preceding block is freed (as some
user-space mallocs do) then it would need to be in a
completely separate cache line.

	David

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