Messages in this thread | | | Date | Wed, 29 Jun 2022 11:43:55 +1200 | Subject | Re: [PATCH v2 3/3] arch/*/: remove CONFIG_VIRT_TO_BUS | From | 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@gmail.com> 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
| |