Re: [PATCH 1/2 v2] dmaengine: at_hdmac: new driver for the Atmel AHB DMA Controller
From: Dan Williams
Date: Wed Jul 01 2009 - 21:18:40 EST
On Fri, Jun 26, 2009 at 3:42 AM, Nicolas Ferre<nicolas.ferre@xxxxxxxxx> wrote:
> This AHB DMA Controller (aka HDMA or DMAC on AT91 systems) is availlable on
> at91sam9rl chip. It will be used on other products in the future.
>
> This first release covers only the memory-to-memory tranfer type. This is the
> only tranfer type supported by this chip. On other products, it will be used
> also for peripheral DMA transfer (slave API support to come).
>
> I used dmatest client without problem in different configurations to test it.
>
> Full documentation for this controller can be found in the SAM9RL datasheet:
> http://www.atmel.com/dyn/products/product_card.asp?part_id=4243
>
> Signed-off-by: Nicolas Ferre <nicolas.ferre@xxxxxxxxx>
> ---
[..]
A quick review comment:
> +/**
> + * atc_desc_get - get a unsused descriptor from free_list
> + * @atchan: channel we want a new descriptor for
> + */
> +static struct at_desc *atc_desc_get(struct at_dma_chan *atchan)
> +{
> + struct at_desc *desc, *_desc;
> + struct at_desc *ret = NULL;
> + unsigned int i = 0;
> + LIST_HEAD(tmp_list);
> +
> + spin_lock_bh(&atchan->lock);
> + list_for_each_entry_safe(desc, _desc, &atchan->free_list, desc_node) {
> + i++;
> + if (async_tx_test_ack(&desc->txd)) {
> + list_del(&desc->desc_node);
> + ret = desc;
> + break;
> + }
> + dev_dbg(chan2dev(&atchan->chan_common),
> + "desc %p not ACKed\n", desc);
> + }
> + spin_unlock_bh(&atchan->lock);
> + dev_vdbg(chan2dev(&atchan->chan_common),
> + "scanned %u descriptors on freelist\n", i);
> +
> + /* no more descriptor available in initial pool : create some more */
> + if (!ret) {
> + for (i = 0; i < INIT_NR_DESCS_PER_CHANNEL; i++) {
> + desc = atc_alloc_descriptor(&atchan->chan_common,
> + GFP_KERNEL);
This cannot be GFP_KERNEL as ->prep_dma_memcpy may be called from an
atomic context. Given that this should only be done in emergency
situations this allocation should probably happen one descriptor at a
time, if it must happen at all. My recommendation is that if you find
that the driver runs out of descriptors on a regular basis increase
INIT_NR_DESCS_PER_CHANNEL and only operate with the descriptors
allocated from atc_alloc_chan_resources(). As it stands this allows
the descriptor list to grow without bounds.
Regards,
Dan
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