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SubjectRe: [PATCH v4 07/12] soc: aspeed: xdma: Add user interface


On Fri, 3 Jan 2020, at 05:57, Eddie James wrote:
> +static ssize_t aspeed_xdma_write(struct file *file, const char __user *buf,
> + size_t len, loff_t *offset)
> +{
> + int rc;
> + struct aspeed_xdma_op op;
> + struct aspeed_xdma_client *client = file->private_data;
> + struct aspeed_xdma *ctx = client->ctx;
> +
> + if (len != sizeof(op))
> + return -EINVAL;
> +
> + rc = copy_from_user(&op, buf, len);
> + if (rc)
> + return rc;
> +
> + if (!op.len || op.len > client->size ||
> + op.direction > ASPEED_XDMA_DIRECTION_UPSTREAM)
> + return -EINVAL;
> +
> + if (file->f_flags & O_NONBLOCK) {
> + if (!mutex_trylock(&ctx->file_lock))
> + return -EAGAIN;
> +
> + if (READ_ONCE(ctx->current_client)) {
> + mutex_unlock(&ctx->file_lock);
> + return -EBUSY;
> + }
> + } else {
> + mutex_lock(&ctx->file_lock);
> +
> + rc = wait_event_interruptible(ctx->wait, !ctx->current_client);
> + if (rc) {
> + mutex_unlock(&ctx->file_lock);
> + return -EINTR;
> + }
> + }
> +
> + aspeed_xdma_start(ctx, &op, client->phys, client);

As aspeed_xdma_start() has to take start_lock, if O_NONBLOCK is set we will
potentially violate its contract if the engine is currently being reset. We could
avoid this by adding

if (READ_ONCE(ctx->in_reset))
return -EBUSY;

before mutex_trylock(&ctx->file_lock) in the O_NONBLOCK path.

Anyway, I think I've convinced myself the locking isn't wrong. It's possible
that it could be improved, but I think we're hitting the point of diminishing
returns.

Andrew

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