Re: [PATCH] parisc: improve cach flushing in arch_sync_dma_for_cpu()

[Helge Deller <deller@gmx.de>]

* John David Anglin <dave.anglin@bell.net>:
> On 2023-05-16 3:09 a.m., Helge Deller wrote:
> > On 5/16/23 00:28, John David Anglin wrote:
> > > On 2023-05-15 2:39 p.m., Helge Deller wrote:
> > > > +    case DMA_BIDIRECTIONAL:
> > > > +        flush_kernel_dcache_range(addr, size);
> > > > +        purge_kernel_dcache_range_asm(addr, addr + size);
> > > I don't think flush and purge are both needed.
> >
> > I'm not sure...
> >
> > Just to fully understand it. Is this short summary correct: ?
> > - flush_kernel_dcache_range: flush cache back to memory, but keep data in cache.
> No.  If present, fdc writes addressed cache line to memory if and only if line is dirty.  It
> then invalidates line.  It does not keep data in cache.
>
> Next read loads from memory.
> >     Next read fetches the data which is still in the cache, thus the next
> >     read doesn't checks if data in memory has been modified in the meantime (e.g. via DMA).
> > - purge_kernel_dcache_range_asm: ignore currently cached data & drop any cached data in that range.
> >     Even if cache has dirty memory which hasn't been written back yet, drop it and don't write back.
> if present, pdc invalidates line.  At privilege level zero, an implementation may optionally write
> back a dirty line to memory.  At non-zero privilege levels, fdc and pdc are effectively the same.
>
> I don't know how to determine whether pdc does write back or not. It would be specified in processor
> ERS.

Thanks for the explanation!
With that, I've attached an updated (untested) patch below.

Helge

[PATCH v2] parisc: improve cach flushing in arch_sync_dma_for_cpu()

Add comment in arch_sync_dma_for_device() and handle the direction flag in
arch_sync_dma_for_cpu().

When receiving data from the device (DMA_FROM_DEVICE) unconditionally
purge the data cache in arch_sync_dma_for_cpu().

Signed-off-by: Helge Deller <deller@gmx.de>
diff --git a/arch/parisc/kernel/pci-dma.c b/arch/parisc/kernel/pci-dma.c
index ba87f791323b..71ed5391f29d 100644
--- a/arch/parisc/kernel/pci-dma.c
+++ b/arch/parisc/kernel/pci-dma.c
@@ -446,11 +446,27 @@ void arch_dma_free(struct device *dev, size_t size, void *vaddr,
 void arch_sync_dma_for_device(phys_addr_t paddr, size_t size,
 		enum dma_data_direction dir)
 {
+	/*
+	 * fdc: The data cache line is written back to memory, if and only if
+	 * it is dirty, and then invalidated from the data cache.
+	 */
 	flush_kernel_dcache_range((unsigned long)phys_to_virt(paddr), size);
 }

 void arch_sync_dma_for_cpu(phys_addr_t paddr, size_t size,
 		enum dma_data_direction dir)
 {
-	flush_kernel_dcache_range((unsigned long)phys_to_virt(paddr), size);
+	unsigned long addr = (unsigned long) phys_to_virt(paddr);
+
+	switch (dir) {
+	case DMA_TO_DEVICE:
+	case DMA_BIDIRECTIONAL:
+		flush_kernel_dcache_range(addr, size);
+		return;
+	case DMA_FROM_DEVICE:
+		purge_kernel_dcache_range_asm(addr, addr + size);
+		return;
+	default:
+		BUG();
+	}
 }