Re: [PATCH 0/2] firmware: cs_dsp: Remove usage of GFP_DMA

[Charles Keepax]

On Wed, Feb 19, 2025 at 04:26:55PM +0000, Richard Fitzgerald wrote:
> On 13/02/2025 3:16 pm, Thomas Weißschuh wrote:
> > On Thu, Feb 13, 2025 at 03:06:59PM +0000, Mark Brown wrote:
> > > On Thu, Feb 13, 2025 at 02:28:06PM +0000, Richard Fitzgerald wrote:
> > > > On 11/02/2025 5:21 pm, Richard Fitzgerald wrote:
> > > 
> > > > > > Not tested on real hardware.
> > > > > > This came up while porting kunit to mips64.
> > > > > > Apparently GFP_DMA does not work there, but IMO the usage of GFP_DMA by
> > > 
> > > > I would say that is a bug in mips64 that should be fixed in mips64.
> > > > It is not reasonable to expect generic drivers to have special cases for
> > > > platforms that don't handle GFP_DMA.
> > 
> > Indeed, I did that, too.
> > 
> > > What specifically is the issue?  If it's a build time issue I'd
> > > definitely agree that we should just be able to assume that platforms at
> > > least build.  IIRC there is a Kconfig you can depend on for DMA but it
> > > seems more trouble than it's worth to fix all users.
> > 
> > More details in [0], It's only a runtime issue.
> > 
> > I'm still wondering how all the on-stack buffers used with regmap_raw_read()
> > and regmap_raw_write() by cs_dsp are satisfying the DMA requirements.
> > 
> There are 3 suspicious regmap_raw_read(). The others are all integers,
> which are guaranteed not to cross a page boundary.
> 
> However, it looks like it might be safe to remove the GFP_DMA flags
> now. regmap_raw_read() calls spi_write_then_read() which specifically
> says that the buffers do not need to be DMA-safe and internally uses a
> DMA-safe buffer. regmap_raw_write() uses either a temporary physically
> contiguous buffer or GFP_DMA buffer (the implementation is terrifyingly
> complex so it's difficult to determine exactly what it does).
> 
> (Some older systems could only use certain special memory areas for DMA
> but we haven't seen any of those used with cs_dsp.)
> 

We also need to consider what the I2C subsystem does, I have a
vague memory of thinking the SPI system will attempt to remap
buffers but I2C will just use them as is. cs_dsp will be used
with both, although SPI is slightly more common for obvious
reasons.

Thanks,
Charles