17.06.2019 13:57, Jon Hunter пишет: > > On 14/06/2019 17:44, Dmitry Osipenko wrote: >> 14.06.2019 18:24, Jon Hunter пишет: >>> >>> On 14/06/2019 16:21, Jon Hunter wrote: >>>> >>>> On 13/06/2019 22:08, Dmitry Osipenko wrote: >>>>> Tegra's APB DMA engine updates words counter after each transferred burst >>>>> of data, hence it can report transfer's residual with more fidelity which >>>>> may be required in cases like audio playback. In particular this fixes >>>>> audio stuttering during playback in a chromiuim web browser. The patch is >>>>> based on the original work that was made by Ben Dooks [1]. It was tested >>>>> on Tegra20 and Tegra30 devices. >>>>> >>>>> [1] https://lore.kernel.org/lkml/20190424162348.23692-1-ben.dooks@xxxxxxxxxxxxxxx/ >>>>> >>>>> Inspired-by: Ben Dooks <ben.dooks@xxxxxxxxxxxxxxx> >>>>> Signed-off-by: Dmitry Osipenko <digetx@xxxxxxxxx> >>>>> --- >>>>> drivers/dma/tegra20-apb-dma.c | 35 ++++++++++++++++++++++++++++------- >>>>> 1 file changed, 28 insertions(+), 7 deletions(-) >>>>> >>>>> diff --git a/drivers/dma/tegra20-apb-dma.c b/drivers/dma/tegra20-apb-dma.c >>>>> index 79e9593815f1..c5af8f703548 100644 >>>>> --- a/drivers/dma/tegra20-apb-dma.c >>>>> +++ b/drivers/dma/tegra20-apb-dma.c >>>>> @@ -797,12 +797,36 @@ static int tegra_dma_terminate_all(struct dma_chan *dc) >>>>> return 0; >>>>> } >>>>> >>>>> +static unsigned int tegra_dma_update_residual(struct tegra_dma_channel *tdc, >>>>> + struct tegra_dma_sg_req *sg_req, >>>>> + struct tegra_dma_desc *dma_desc, >>>>> + unsigned int residual) >>>>> +{ >>>>> + unsigned long status, wcount = 0; >>>>> + >>>>> + if (!list_is_first(&sg_req->node, &tdc->pending_sg_req)) >>>>> + return residual; >>>>> + >>>>> + if (tdc->tdma->chip_data->support_separate_wcount_reg) >>>>> + wcount = tdc_read(tdc, TEGRA_APBDMA_CHAN_WORD_TRANSFER); >>>>> + >>>>> + status = tdc_read(tdc, TEGRA_APBDMA_CHAN_STATUS); >>>>> + >>>>> + if (!tdc->tdma->chip_data->support_separate_wcount_reg) >>>>> + wcount = status; >>>>> + >>>>> + if (status & TEGRA_APBDMA_STATUS_ISE_EOC) >>>>> + return residual - sg_req->req_len; >>>>> + >>>>> + return residual - get_current_xferred_count(tdc, sg_req, wcount); >>>>> +} >>>>> + >>>>> static enum dma_status tegra_dma_tx_status(struct dma_chan *dc, >>>>> dma_cookie_t cookie, struct dma_tx_state *txstate) >>>>> { >>>>> struct tegra_dma_channel *tdc = to_tegra_dma_chan(dc); >>>>> + struct tegra_dma_sg_req *sg_req = NULL; >>>>> struct tegra_dma_desc *dma_desc; >>>>> - struct tegra_dma_sg_req *sg_req; >>>>> enum dma_status ret; >>>>> unsigned long flags; >>>>> unsigned int residual; >>>>> @@ -838,6 +862,8 @@ static enum dma_status tegra_dma_tx_status(struct dma_chan *dc, >>>>> residual = dma_desc->bytes_requested - >>>>> (dma_desc->bytes_transferred % >>>>> dma_desc->bytes_requested); >>>>> + residual = tegra_dma_update_residual(tdc, sg_req, dma_desc, >>>>> + residual); >>>> >>>> I had a quick look at this, I am not sure that we want to call >>>> tegra_dma_update_residual() here for cases where the dma_desc is on the >>>> free_dma_desc list. In fact, couldn't this be simplified a bit for case >>>> where the dma_desc is on the free list? In that case I believe that the >>>> residual should always be 0. >>> >>> Actually, no, it could be non-zero in the case the transfer is aborted. >> >> Looks like everything should be fine as-is. > > I am still not sure we want to call this for the case where dma_desc is > on the free list. You're right! It's a bug there! The sg_req=NULL if dma_desc is on the free list, hence it will result in a NULL dereference. I'll fix it in v2 and will avoid the offending call, like you're suggesting. >> BTW, it's a bit hard to believe that there is any real benefit from the >> free_dma_desc list at all, maybe worth to just remove it? > > I think you need to elaborate a bit more here. I am not a massive fan of > this driver, but I am also not in the mood for changing unless there is > a good reason. It looks like the whole point of the free list is to have a cache of preallocated dma_desc's, but dma_desc allocation and initialization doesn't cost anything in comparison to the free list because memory is allocated from a SLAB cache and then the initialization will happen on CPU's cache. So the free list is quite pointless in terms of optimization. Moreover what if driver allocates a lot of dma_desc's and uses them just once? Looks like it will be quite a lot of wasted memory on the free list.
