linux-kernel - Re: [PATCH 07/13] dmaengine: introduce dma_request

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Date:	Mon, 15 Dec 2008 16:55:06 -0700
From:	"Dan Williams" <dan.j.williams@...el.com>
To:	"Sosnowski, Maciej" <maciej.sosnowski@...el.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"hskinnemoen@...el.com" <hskinnemoen@...el.com>,
	"g.liakhovetski@....de" <g.liakhovetski@....de>,
	"nicolas.ferre@...el.com" <nicolas.ferre@...el.com>
Subject: Re: [PATCH 07/13] dmaengine: introduce dma_request_channel and private channels

On Fri, Dec 12, 2008 at 7:29 AM, Sosnowski, Maciej
<maciej.sosnowski@...el.com> wrote:
>>       clear_bit(DMA_INTERRUPT, dma_cap_mask_all.bits);
>> +     clear_bit(DMA_PRIVATE, dma_cap_mask_all.bits);
>>       clear_bit(DMA_SLAVE, dma_cap_mask_all.bits);
>
> The comment above should be updated according to this change:
> -/* 'interrupt' and 'slave' are channel capabilities, but are not
> +/* 'interrupt', 'private' and 'slave' are channel capabilities, but are not
>

ok.

>> +static struct dma_chan *private_candidate(dma_cap_mask_t *mask,
>> struct dma_device *dev) +{
>> +     struct dma_chan *chan;
>> +     struct dma_chan *ret = NULL;
>> +
>> +     /* devices with multiple channels need special handling as we need
>> to +   * ensure that all channels are either private or public.
>> +      */
>> +     if (dev->chancnt > 1 && !dma_has_cap(DMA_PRIVATE, dev->cap_mask))
>> +             list_for_each_entry(chan, &dev->channels, device_node) {
>> +                     /* some channels are already publicly allocated */
>> +                     if (chan->client_count)
>> +                             return NULL;
>> +             }
>
> Isn't it a dangerous approach to let clients consume for their exclusive usage channels
> meant for general-purpose ("pubilc" ones)?
> Why not to limit private_candidate to devices with DMA_PRIVATE capability only?
>

This allows unused channels to be claimed by dma_request_channel().
It is not dangerous as long as ->client_count is zero.

>> +
>> +     list_for_each_entry(chan, &dev->channels, device_node) {
>> +             if (!__dma_chan_satisfies_mask(chan, mask)) {
>> +                     pr_debug("%s: %s wrong capabilities\n",
>> +                              __func__, dev_name(&chan->dev));
>> +                     continue;
>> +             }
>
> As capabilities are per device, this check could be performed just once
> before list_for_each_entry(chan, &dev->channels, device_node).
>

Yes, changed.

Thanks,
Dan
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