linux-kernel - Re: [PATCH] dmaengine: device must have at least one channel

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Date:	Wed, 27 Jul 2016 21:48:24 -0700
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Vinod Koul <vinod.koul@...el.com>
Cc:	Dan Williams <dan.j.williams@...el.com>,
	linaro-kernel@...ts.linaro.org,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	dmaengine@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] dmaengine: device must have at least one channel

On 28-07-16, 09:18, Vinod Koul wrote:
> On Wed, Jul 27, 2016 at 02:32:58PM -0700, Viresh Kumar wrote:
> > The DMA device can't be registered if it doesn't have any channels
> > registered at all. Moreover, it leads to memory leak and is reported by
> > kmemleak as (on 3.10 kernel, and same shall happen on mainline):
> > 
> > unreferenced object 0xffffffc09e597240 (size 64):
> >   comm "swapper/0", pid 1, jiffies 4294877736 (age 7060.280s)
> >   hex dump (first 32 bytes):
> >     00 00 00 00 c0 ff ff ff 30 00 00 ff 00 00 00 ff  ........0.......
> >     00 00 00 ff 00 00 00 ff 00 00 00 ff 00 00 00 ff  ................
> >   backtrace:
> >     [<ffffffc0003079ec>] create_object+0x148/0x2a0
> >     [<ffffffc000cc150c>] kmemleak_alloc+0x80/0xbc
> >     [<ffffffc000303a7c>] kmem_cache_alloc_trace+0x120/0x1ac
> >     [<ffffffc00054771c>] dma_async_device_register+0x160/0x46c
> >     [<ffffffc000548958>] foo_probe+0x1a0/0x264
> >     [<ffffffc0005d6658>] platform_drv_probe+0x14/0x20
> >     [<ffffffc0005d50cc>] driver_probe_device+0x160/0x374
> >     [<ffffffc0005d538c>] __driver_attach+0x60/0x90
> >     [<ffffffc0005d3e78>] bus_for_each_dev+0x7c/0xb0
> >     [<ffffffc0005d4a0c>] driver_attach+0x1c/0x28
> >     [<ffffffc0005d459c>] bus_add_driver+0x124/0x248
> >     [<ffffffc0005d59cc>] driver_register+0x90/0x110
> >     [<ffffffc0005d6bf4>] platform_driver_register+0x58/0x64
> >     [<ffffffc00142a70c>] foo_driver_init+0x10/0x1c
> >     [<ffffffc000200878>] do_one_initcall+0xac/0x148
> >     [<ffffffc00140096c>] kernel_init_freeable+0x1a0/0x258
> > 
> > Return -ENODEV from dma_async_device_register() on such a case.
> > 
> > /Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>
> > ---
> > Hi Vinod,
> > 
> > Sorry if the fundamentals behind this patch are completely incorrect,
> > i.e. We *can't* register a dma device with 0 channels. Its been long
> > that I have worked on dma stuff :)
> 
> This sounds okay,

Great..

> but why would anyone register a device without having
> a channel?

A buggy driver? The core should be handling this no matter what,
right?

The out of tree driver, 3.10 based driver I have looks to be buggy to
me. It is doing something like this (please ignore naming/formatting):

xlate()
{
        chan = alloc();
        list_add(chan.device_node, &dma_dev.channels)
}

probe()
{
        ...
        dma_async_device_register()
        of_dma_controller_register(xlate);
        ...
}

But with this it looks like the 'chan' structure wouldn't get
initialized properly as that is done from within
dma_async_device_register(). Right? Also, I am not sure if DMA is
getting used at all right now on my platform. Just noticed the
memleak, which I tried to fix.

-- 
viresh