Re: [PATCH v1 1/3] dma: tegra: finer granularity residual for tx_status

Stephen Warren
2014-05-07T16:37:33Z

On 05/06/2014 03:22 PM, Christopher Freeman wrote:
> Get word-level granularity from hardware for calculating
> the transfer count remaining.

> diff --git a/drivers/dma/tegra20-apb-dma.c b/drivers/dma/tegra20-apb-dma.c

> +static int tegra_dma_wcount_in_bytes(struct dma_chan *dc)

A lot of the code in this function is identical to the code in
tegra_dma_terminate_all() which does the same thing. Can this be pulled
out into a shared utility function?

> + tegra_dma_pause(tdc, true);

Is this continual pausing/resuming of the DMA operation going to
negatively affect performance?

> + /* in case of interrupt, handle it and don't read wcount reg */
> + status = tdc_read(tdc, TEGRA_APBDMA_CHAN_STATUS);
> + if (status & TEGRA_APBDMA_STATUS_ISE_EOC) {
> + tdc_write(tdc, TEGRA_APBDMA_CHAN_STATUS, status);
> + dev_info(tdc2dev(tdc), "%s():handling isr\n", __func__);

If you swap the order of patches 1 and 2, then you can just add that
line as dev_dbg() from the start, and you won't need to change it in the
next patch.

> + tdc->isr_handler(tdc, false);
> + tegra_dma_resume(tdc);
> + return 0;

Why resume and return here? Shouldn't those last 2 lines be removed, so
the code can simply continue through the balance of the function and
return the actual status. tegra_dma_terminate_all() does that.

> @@ -812,9 +851,22 @@ static enum dma_status tegra_dma_tx_status(struct dma_chan *dc,
> list_for_each_entry(sg_req, &tdc->pending_sg_req, node) {
> dma_desc = sg_req->dma_desc;
> if (dma_desc->txd.cookie == cookie) {
> + hw_byte_count = tegra_dma_wcount_in_bytes(dc);
> +
> + if (!list_empty(&tdc->pending_sg_req))

Since this code is inside a loop that iterates over tha list, I don't
think the list can ever be empty.

> + first_entry =
> + list_first_entry(&tdc->pending_sg_req,
> + typeof(*first_entry), node);
> +
> residual = dma_desc->bytes_requested -
> (dma_desc->bytes_transferred %
> dma_desc->bytes_requested);
> +
> + /* hw byte count only applies to current transaction */
> + if (first_entry &&
> + first_entry->dma_desc->txd.cookie == cookie)
> + residual -= hw_byte_count;
> +
> dma_set_residue(txstate, residual);

Why not re-order the added code so that all the new code is added in one
place, and the hw_byte_count value is only calculated if it's used, i.e.:

residual = ...;
first_entry = ...;
if (sg_reg == first_entry) {
hw_byte_count = ...;
residual -= hw_byte_count;
}