Re: [PATCH v2 3/3] arch/*/: remove CONFIG_VIRT_TO_BUS

[Michael Schmitz]

Hi Arnd,

On 29/06/22 09:55, Arnd Bergmann wrote:
> On Tue, Jun 28, 2022 at 11:38 PM Michael Schmitz <schmitzmic@xxxxxxxxx> wrote:
> > On 28/06/22 19:08, Arnd Bergmann wrote:
> > > I see two other problems with your patch though:
> > >
> > > a) you still duplicate the cache handling: the cache_clear()/cache_push()
> > > is supposed to already be done by dma_map_single() when the device
> > > is not cache-coherent.
> > That's one of the 'liberties' I alluded to. The reason I left these in
> > is that I'm none too certain what device feature the DMA API uses to
> > decide a device isn't cache-coherent. If it's dev->coherent_dma_mask,
> > the way I set up the device in the a3000 driver should leave the
> > coherent mask unchanged. For the Zorro drivers, devices are set up to
> > use the same storage to store normal and coherent masks - something we
> > most likely want to change. I need to think about the ramifications of
> > that.
> >
> > Note that zorro_esp.c uses dma_sync_single_for_device() and uses a 32
> > bit coherent DMA mask which does work OK. I might  ask Adrian to test a
> > change to only set dev->dma_mask, and drop the
> > dma_sync_single_for_device() calls if there's any doubt on this aspect.
> The "coherent_mask" is independent of the cache flushing. On some
> architectures, a device can indicate whether it needs cache management
> or not to guarantee coherency, but on m68k it appears that we always
> assume it does, see arch/m68k/kernel/dma.c

Thanks - what I see there indicates that on the relevant platforms, 
pages mapped for DMA have their page table cache bits modified to make 
them non-cacheable (and I suppose unmapping restores the default cache 
bits). That means I should use dma_set_mask_and_coherent() here to take 
advantage of this, and no need to mess around with 
dma_sync_single_for_device() in the drivers' dma_setup() functions.

> > > b) The bounce buffer is never mapped here, instead you have the
> > > virt_to_phys() here, which is not the same. I think you need to map
> > > the pointer that actually gets passed down to the device after deciding
> > > to use a bouce buffer or not.
> > I hadn't realized that I can map the bounce buffer just as it's done for
> > the SCp data buffer. Should have been obvious, but I'm still learning
> > about the DMA API.
> >
> > I've updated the patch now, will re-send as part of a complete series
> > once done.
> I suppose you can just drop the bounce buffer if this just comes
> from kmalloc().

That's only true for a3000 and mvme147 though.

Cheers,

    Michael

>         Arnd