[RFCv2][PATCH 2/5] arm: Implement ARCH_HAS_FORCE_CACHE
Laura Abbott
labbott at redhat.com
Tue Aug 16 13:39:48 PDT 2016
On 08/10/2016 04:22 PM, Russell King - ARM Linux wrote:
> On Mon, Aug 08, 2016 at 10:49:34AM -0700, Laura Abbott wrote:
>> +/*
>> + * Make an area consistent for devices.
>> + * Note: Drivers should NOT use this function directly, as it will break
>> + * platforms with CONFIG_DMABOUNCE.
>> + * Use the driver DMA support - see dma-mapping.h (dma_sync_*)
>> + */
>> +void __dma_page_cpu_to_dev(struct page *page, unsigned long off,
>> + size_t size, enum dma_data_direction dir)
>> +{
>> + phys_addr_t paddr;
>> +
>> + dma_cache_maint_page(page, off, size, dir, dmac_map_area);
>> +
>> + paddr = page_to_phys(page) + off;
>> + if (dir == DMA_FROM_DEVICE) {
>> + outer_inv_range(paddr, paddr + size);
>> + } else {
>> + outer_clean_range(paddr, paddr + size);
>> + }
>> + /* FIXME: non-speculating: flush on bidirectional mappings? */
>> +}
>> +
>> +void __dma_page_dev_to_cpu(struct page *page, unsigned long off,
>> + size_t size, enum dma_data_direction dir)
>> +{
>> + phys_addr_t paddr = page_to_phys(page) + off;
>> +
>> + /* FIXME: non-speculating: not required */
>> + /* in any case, don't bother invalidating if DMA to device */
>> + if (dir != DMA_TO_DEVICE) {
>> + outer_inv_range(paddr, paddr + size);
>> +
>> + dma_cache_maint_page(page, off, size, dir, dmac_unmap_area);
>> + }
>> +
>> + /*
>> + * Mark the D-cache clean for these pages to avoid extra flushing.
>> + */
>> + if (dir != DMA_TO_DEVICE && size >= PAGE_SIZE) {
>> + unsigned long pfn;
>> + size_t left = size;
>> +
>> + pfn = page_to_pfn(page) + off / PAGE_SIZE;
>> + off %= PAGE_SIZE;
>> + if (off) {
>> + pfn++;
>> + left -= PAGE_SIZE - off;
>> + }
>> + while (left >= PAGE_SIZE) {
>> + page = pfn_to_page(pfn++);
>> + set_bit(PG_dcache_clean, &page->flags);
>> + left -= PAGE_SIZE;
>> + }
>> + }
>> +}
>
> I _really_ don't want these exposed in any shape or form to driver
> code. I've seen too many hacks out there where people have gone
> under the cover of the APIs they should be using, and headed straight
> for the low-level functionality - adding function prototypes to get
> at stuff they have no business doing. Moving this here is just
> asking for it to be abused.
>
>> +
>> +void kernel_force_cache_clean(struct page *page, size_t size)
>> +{
>> + __dma_page_cpu_to_dev(page, 0, size, DMA_BIDIRECTIONAL);
>> +}
>> +
>> +void kernel_force_cache_invalidate(struct page *page, size_t size)
>> +{
>> + __dma_page_dev_to_cpu(page, 0, size, DMA_BIDIRECTIONAL);
>> +}
>
> Nothing in our implementation of these DMA operations guarantees
> that those mean "clean" and "invalidate". The DMA operations are
> there so that CPUs can implement whatever they need at the map and
> unmap times - and I've been very careful not to specify which cache
> operations are involved.
>
> For example, on older CPUs, __dma_page_dev_to_cpu() is almost
> always a no-op.
>
> If you want something that does something specific, then we need
> something designed to do something specific. Please don't re-use
> what you think will fit.
>
I see what you are saying. What I really wanted was to re-use some of
the code that dma_cache_maint_page was doing for highmem handling
but it looks like I picked the wrong layer to make common. I'll give
this some thought.
Thanks,
Laura
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