linux-kernel - Re: [PATCH] dma-coherent: add support for multi coherent rmems per dev

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Date: Thu, 1 Feb 2024 07:41:31 -0800
From: Greg KH <gregkh@...uxfoundation.org>
To: Robin Murphy <robin.murphy@....com>
Cc: Howard Yen <howardyen@...gle.com>, hch@....de, m.szyprowski@...sung.com,
	andriy.shevchenko@...ux.intel.com, petr.tesarik.ext@...wei.com,
	rafael@...nel.org, broonie@...nel.org, james@...iv.tech,
	james.clark@....com, masahiroy@...nel.org,
	linux-kernel@...r.kernel.org, iommu@...ts.linux.dev
Subject: Re: [PATCH] dma-coherent: add support for multi coherent rmems per
 dev

On Thu, Feb 01, 2024 at 10:45:30AM +0000, Robin Murphy wrote:
> On 2024-02-01 9:35 am, Howard Yen wrote:
> > Add support for multiple coherent rmems per device. This patch addes
> > dma_mem_list to device structure to store multiple rmems.
> > 
> > These multiple rmems can be assigned to the device one by one by
> > of_reserved_mem_device_init_by_idx() with the memory-region
> > declaration in device tree as below and store the rmem to the dma_mem_list.
> > 
> > 	device1@0 {
> > 		...
> > 		memory-region = <&reserved_mem0>, <&reserved_mem1>;
> > 		...
> > 	};
> > 
> > When driver tries to allocate memory from the rmems, looks for the first
> > available rmem and allocates the memory from this rmem.
> > 
> > Then if driver removed, of_reserved_mem_device_release() needs to be
> > invoked to release all the rmems assigned to the device.
> > 
> > Signed-off-by: Howard Yen <howardyen@...gle.com>
> > ---
> >   include/linux/device.h |  1 +
> >   kernel/dma/coherent.c  | 66 +++++++++++++++++++++++++++++++++++-------
> >   2 files changed, 56 insertions(+), 11 deletions(-)
> > 
> > diff --git a/include/linux/device.h b/include/linux/device.h
> > index 97c4b046c09d..c8682ee507cf 100644
> > --- a/include/linux/device.h
> > +++ b/include/linux/device.h
> > @@ -751,6 +751,7 @@ struct device {
> >   #ifdef CONFIG_DMA_DECLARE_COHERENT
> >   	struct dma_coherent_mem	*dma_mem; /* internal for coherent mem
> >   					     override */
> > +	struct list_head	dma_mem_list;
> 
> I'm not necessarily against the idea, but only if it's implemented sensibly.
> If we're going to have a list of these it should *replace* the existing
> pointer, not do this weird thing with both.

Agreed, it should be one pointer max for this structure for this type of
thing.  Why not move it into the dma_coherent_mem structure?

thanks,

greg k-h