1 files changed, 25 insertions, 2 deletions

diff --git a/sound/pci/es1938.c b/sound/pci/es1938.c
index fbe3da73eaff..1a314fa99c45 100644
--- a/sound/pci/es1938.c
+++ b/sound/pci/es1938.c
@@ -226,6 +226,7 @@ struct es1938 {
         unsigned int dma2_start;
         unsigned int dma1_shift;
         unsigned int dma2_shift;
+        unsigned int last_capture_dmaaddr;
         unsigned int active;
 
         spinlock_t reg_lock;
@@ -528,6 +529,7 @@ static void snd_es1938_capture_setdma(struct es1938 *chip)
         outb(1, SLDM_REG(chip, DMAMASK));
         outb(0x14, SLDM_REG(chip, DMAMODE));
         outl(chip->dma1_start, SLDM_REG(chip, DMAADDR));
+        chip->last_capture_dmaaddr = chip->dma1_start;
         outw(chip->dma1_size - 1, SLDM_REG(chip, DMACOUNT));
         /* 3. Unmask DMA */
         outb(0, SLDM_REG(chip, DMAMASK));
@@ -769,19 +771,40 @@ static int snd_es1938_playback_prepare(struct snd_pcm_substream *substream)
         return -EINVAL;
 }
 
+/* during the incrementing of dma counters the DMA register reads sometimes
+   returns garbage. To ensure a valid hw pointer, the following checks which
+   should be very unlikely to fail are used:
+   - is the current DMA address in the valid DMA range ?
+   - is the sum of DMA address and DMA counter pointing to the last DMA byte ?
+   One can argue this could differ by one byte depending on which register is
+   updated first, so the implementation below allows for that.
+*/
 static snd_pcm_uframes_t snd_es1938_capture_pointer(struct snd_pcm_substream *substream)
 {
         struct es1938 *chip = snd_pcm_substream_chip(substream);
         size_t ptr;
+#if 0
         size_t old, new;
-#if 1
         /* This stuff is *needed*, don't ask why - AB */
         old = inw(SLDM_REG(chip, DMACOUNT));
         while ((new = inw(SLDM_REG(chip, DMACOUNT))) != old)
                 old = new;
         ptr = chip->dma1_size - 1 - new;
 #else
-        ptr = inl(SLDM_REG(chip, DMAADDR)) - chip->dma1_start;
+        size_t count;
+        unsigned int diff;
+
+        ptr = inl(SLDM_REG(chip, DMAADDR));
+        count = inw(SLDM_REG(chip, DMACOUNT));
+        diff = chip->dma1_start + chip->dma1_size - ptr - count;
+
+        if (diff > 3 || ptr < chip->dma1_start
+              || ptr >= chip->dma1_start+chip->dma1_size)
+          ptr = chip->last_capture_dmaaddr;            /* bad, use last saved */
+        else
+          chip->last_capture_dmaaddr = ptr;            /* good, remember it */
+
+        ptr -= chip->dma1_start;
 #endif
         return ptr >> chip->dma1_shift;
 }