Re: [PATCH 3/3] spi: rockchip: check requesting dma channel with EPROBE_DEFER

From: Doug Anderson
Date: Mon Mar 21 2016 - 19:33:40 EST


Shawn,

On Wed, Mar 9, 2016 at 12:11 AM, Shawn Lin <shawn.lin@xxxxxxxxxxxxxx> wrote:
> Let's defer probing the driver if the return value of
> dma_request_slave_channel is ERR_PTR(-EPROBE_DEFER) instead
> of disabling dma capability directly.
>
> Signed-off-by: Shawn Lin <shawn.lin@xxxxxxxxxxxxxx>
> ---
>
> drivers/spi/spi-rockchip.c | 8 +++++++-
> 1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/spi/spi-rockchip.c b/drivers/spi/spi-rockchip.c
> index ca4f4e0..75fa990 100644
> --- a/drivers/spi/spi-rockchip.c
> +++ b/drivers/spi/spi-rockchip.c
> @@ -737,8 +737,14 @@ static int rockchip_spi_probe(struct platform_device *pdev)
> master->handle_err = rockchip_spi_handle_err;
>
> rs->dma_tx.ch = dma_request_slave_channel(rs->dev, "tx");
> - if (!rs->dma_tx.ch)
> + if (IS_ERR_OR_NULL(rs->dma_tx.ch)) {
> + /* Check tx to see if we need defer probing driver */
> + if (PTR_ERR(rs->dma_tx.ch) == -EPROBE_DEFER) {
> + ret = -EPROBE_DEFER;
> + goto err_get_fifo_len;
> + }
> dev_warn(rs->dev, "Failed to request TX DMA channel\n");

Presumably Dan would be happy if you just add this right after the dev_warn():
rs->dma_tx.ch = NULL;

Presumably from Dan's email it would also be wise to make sure you
don't pass NULL to PTR_ERR, which you could probably do by just using
ERR_PTR instead of PTR_ERR. I think you could structure like this:

rs->dma_tx.ch = dma_request_slave_channel(rs->dev, "tx");
- if (!rs->dma_tx.ch)
+ if (IS_ERR_OR_NULL(rs->dma_tx.ch)) {
+ /* Check tx to see if we need defer probing driver */
+ if (rs->dma_tx.ch == ERR_PTR(-EPROBE_DEFER)) {
+ ret = -EPROBE_DEFER;
+ goto err_get_fifo_len;
+ }
dev_warn(rs->dev, "Failed to request TX DMA channel\n");
+ rs->dma_tx.ch = NULL;
+ }


With that change your patch should be happy, I think. If some new
unknown error return gets added to dma_request_slave_channel() then
your code will continue to work properly. Such a change is simple and
safe, so presumably you could just spin your patch with that fix.
Although unlikely, it's probably good to check for IS_ERR_OR_NULL()
when requesting the "rx" channel too.

...but, looking at this, presumably before landing any patch that made
dma_request_slave_channel() return -EPROBE_DEFER you'd need to modify
_all_ users of dma_request_slave_channel to handle error pointers
being returned. Right now dma_request_slave_channel() says it returns
a pointer to a channel or NULL and the function explicitly avoids
returning any errors. That might be possible, but it's a big
change...


-Doug