From: Krzysztof Helt Date: Sun, 11 Oct 2009 10:48:00 +0000 (+0200) Subject: ALSA: snd_dma_pointer workaround for chipsets with buggy DMA X-Git-Tag: v2.6.33-rc1~77^2~16^2~39 X-Git-Url: http://review.tizen.org/git/?a=commitdiff_plain;h=8066e51ae7329220f459470a38387f8533e99141;p=platform%2Fkernel%2Flinux-3.10.git ALSA: snd_dma_pointer workaround for chipsets with buggy DMA The chipsets with the isa_dma_bridge_buggy set do not stop DMA during DMA counter reads. The DMA counter is read in two 8-bit read steps on x86 platform. Sometimes, such reads happen during higher byte change so the lower byte is already decremented (rolled over) but the higher byte is not. It introduces an error that position is moved 256 bytes ahead of the true position. Thus, the next DMA position read can return a lower value then the previous read. If the DMA position is decreased (reversed) the ALSA subsystem is tricked into the playback underrun error and resets the playback. It results in a "pop" during a playback. Work around the issue by reading the counter twice and choosing a higher value. Signed-off-by: Krzysztof Helt Signed-off-by: Takashi Iwai --- diff --git a/sound/core/isadma.c b/sound/core/isadma.c index 79f0f16..950e19b 100644 --- a/sound/core/isadma.c +++ b/sound/core/isadma.c @@ -85,16 +85,24 @@ EXPORT_SYMBOL(snd_dma_disable); unsigned int snd_dma_pointer(unsigned long dma, unsigned int size) { unsigned long flags; - unsigned int result; + unsigned int result, result1; flags = claim_dma_lock(); clear_dma_ff(dma); if (!isa_dma_bridge_buggy) disable_dma(dma); result = get_dma_residue(dma); + /* + * HACK - read the counter again and choose higher value in order to + * avoid reading during counter lower byte roll over if the + * isa_dma_bridge_buggy is set. + */ + result1 = get_dma_residue(dma); if (!isa_dma_bridge_buggy) enable_dma(dma); release_dma_lock(flags); + if (unlikely(result < result1)) + result = result1; #ifdef CONFIG_SND_DEBUG if (result > size) snd_printk(KERN_ERR "pointer (0x%x) for DMA #%ld is greater than transfer size (0x%x)\n", result, dma, size);