Re: Do Qualcomm drivers use DMA buffers for request_firmware_into_buf()?

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Thu, Jun 07, 2018 at 06:43:05PM +0200, Ard Biesheuvel wrote:
> On 7 June 2018 at 18:33, Greg Kroah-Hartman <gregkh@linuxfoundation.org> wrote:
> > On Thu, Jun 07, 2018 at 06:23:01PM +0200, Ard Biesheuvel wrote:
> >> On 7 June 2018 at 18:18, Bjorn Andersson <bjorn.andersson@linaro.org> wrote:
> >> > On Wed 06 Jun 13:32 PDT 2018, Luis R. Rodriguez wrote:
> >> >
> >> >> On Fri, Jun 01, 2018 at 09:23:46PM +0200, Luis R. Rodriguez wrote:
> >> >> > On Tue, May 08, 2018 at 03:38:05PM +0000, Luis R. Rodriguez wrote:
> >> >> > > On Fri, May 04, 2018 at 12:44:37PM -0700, Martijn Coenen wrote:
> >> >> > > >
> >> >> > > > I think the Qualcomm folks owning this (Andy, David, Bjorn, already
> >> >> > > > cc'd here) are better suited to answer that question.
> >> >> > >
> >> >> > > Andy, David, Bjorn?
> >> >> >
> >> >> > Andy, David, Bjorn?
> >> >>
> >> >> A month now with no answer...
> >> >>
> >> >
> >> > The patch at the top of this thread doesn't interest me and you didn't
> >> > bother sending your question To me.
> >> >
> >> > As a matter of fact I'm confused to what the actual question is.
> >> >
> >>
> >> The actual question is whether it is really required that the firmware
> >> is loaded by the kernel into a buffer that is already mapped for DMA
> >> at that point, and thus accessible by the device.
> >>
> >> To me, it is not entirely clear what the nature is of the firmware
> >> that we are talking about, since it seems to be getting passed to the
> >> secure world as well?
> >>
> >> In any case, the preferred model in terms of validation/sig checking is
> >>
> >> 1) allocate a CPU accessible buffer
> >>
> >> 2) request the firmware into it (which may include a sig check under the hood)
> >>
> >> 3) map the buffer for DMA to the device so it can load the firmware.
> >>
> >> 4) kick off the DMA transfer.
> >>
> >> The use of dma_alloc_coherent() for this purpose seems unnecessary,
> >> given that the DMA transfer is not bidirectional. Would it be possible
> >> to replace it with something like the above sequence?
> >
> > Why not just use kmalloc, it will always return a DMAable buffer.
> >
> 
> On a PC maybe. But there are plenty of systems where bidirectional DMA
> mappings require uncached memory (i.e., if the device doesn't snoop
> the caches), in which case a kmalloc'ed buffer is useless.
> dma_alloc_coherent() hides the platform constraints from the driver,
> so it is a very useful abstraction for this use case.

kmalloc should always return a DMAble buffer.  If that is not true, we
have a _LOT_ of broken drivers.  What systems is this not true on, and
how are you running USB on them?  :)

thanks,

greg k-h