[alsa-devel] 答复: 答复: A bug about cache inconsistency report

[Hans Hu(SH-RD)]

> > >Then the next step would be to fake sg-buffer from this straight
> > >buffer.  Revert the above, and modify sgbuf.c to the following:
> > >- Allocate a large continuous buffer
> > >- Assign each page in this large buffer
> >
> > >If this still works, it's not about vmap, but it just means that
> > >the physically ordered pages do matter -- implicitly showing that
> > >the snooping behavior isn't properly turned on / off on the controller.
> >
> > To fake SG-buffer, I did this test: restore all the codes to the original, then added some codes in snd_malloc_sgbuf_pages() like below, the result is badly niose.
>
> >Not really what I meant.  Rather something like below (totally untested).
>
> [Hans :] I know what you mean now and complete code like below, but the result is still noise.

>OK, so indeed the vmapped address does seem matter.  Interesting.
>What kind of user access does produce the noise?  Does it via aplay mmap mode, too?

>In anyway, if the vmap is a problem, it might be worked around a patch like below (again totally untested and not sure whether it's correct).

[Hans :] As I know, In the hardware layer HDAC’s stream have two data transport path : non-snoop & snoop; In the software layer ALSA-Driver have two data transport path : mmap & not mmap(test shows, it is dependent on wav's format or mmap_flag in aplay.c). When hardware at non-snoop mode, without hardware module's help, software must mark the mem to uncacheable type: when mmap used, the mark action happened at pcm_mmap_prepare(), the not-mmap mode's mark action happened at __mark_pages_wc(). And when at not-mmap mode, the vmapped address directly used in snd_pcm_lib_write_transfer() -> copy_from_user().
Yes, of course, this path combinations(non-snoop + not-mmap) unlikely be used by most music players.

[Hans :] I complete the patch in your last mail like below, not used the vunmap & re-vmap way(it has a little problem). Result is OK now(no noise anymore).

--- a/sound/pci/hda/hda_intel.c
+++ b/sound/pci/hda/hda_intel.c
#ifdef CONFIG_X86
+#ifdef CONFIG_SND_DMA_SGBUF
+#include <linux/vmalloc.h>
+#include <linux/sched.h>
+#include <asm/paravirt.h>
+
+static int bad_address(void *p)
+{
+unsigned long dummy;
+
+return probe_kernel_address((unsigned long *)p, dummy);
+}
+
+static pte_t * dump_pte(unsigned long address)
+{
+pgd_t *base = __va(read_cr3() & PHYSICAL_PAGE_MASK);
+pgd_t *pgd = base + pgd_index(address);
+pud_t *pud;
+pmd_t *pmd;
+pte_t *pte;
+
+if (bad_address(pgd))
+goto bad;
+
+if (!pgd_present(*pgd))
+goto bad;
+
+pud = pud_offset(pgd, address);
+if (bad_address(pud))
+goto bad;
+
+if (!pud_present(*pud) || pud_large(*pud))
+goto bad;
+
+pmd = pmd_offset(pud, address);
+if (bad_address(pmd))
+goto bad;
+
+if (!pmd_present(*pmd) || pmd_large(*pmd))
+goto bad;
+
+pte = pte_offset_kernel(pmd, address);
+if (bad_address(pte))
+goto bad;
+
+return pte;
+bad:
+return NULL;
+}
+#endif
 static void __mark_pages_wc(struct azx *chip, struct snd_dma_buffer *dmab, bool on)
 {
 int pages;
@@ -410,12 +459,28 @@ static void __mark_pages_wc(struct azx *chip, struct snd_dma_buffer *dmab, bool
 #ifdef CONFIG_SND_DMA_SGBUF
 if (dmab->dev.type == SNDRV_DMA_TYPE_DEV_SG) {
 struct snd_sg_buf *sgbuf = dmab->private_data;
+pgprot_t prot;
+int i;
+unsigned long addr;
+pte_t * pte;
 if (chip->driver_type == AZX_DRIVER_CMEDIA)
 return; /* deal with only CORB/RIRB buffers */
-if (on)
+if (on){
 set_pages_array_wc(sgbuf->page_table, sgbuf->pages);
-else
+prot = pgprot_writecombine(PAGE_KERNEL);
+}
+else{
 set_pages_array_wb(sgbuf->page_table, sgbuf->pages);
+prot = PAGE_KERNEL;
+}
+for (i = 0; i < sgbuf->pages; i ++)
+{
+addr = (unsigned long)dmab->area + PAGE_SIZE * i;
+pte = dump_pte(addr);
+if (pte)
+set_pte_at(&init_mm, addr, pte, mk_pte(sgbuf->page_table[i], prot));
+else WARN_ON(1);
+}
 return;
 }
 #endif


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