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From:   Claire Chang <tientzu@chromium.org>
Date:   Thu, 14 Jan 2021 17:06:07 +0800
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Subject: Re: [RFC PATCH v3 2/6] swiotlb: Add restricted DMA pool
To:     Christoph Hellwig <hch@lst.de>
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On Wed, Jan 13, 2021 at 8:42 PM Christoph Hellwig <hch@lst.de> wrote:
>
> > +#ifdef CONFIG_SWIOTLB
> > +     struct io_tlb_mem       *dma_io_tlb_mem;
> >  #endif
>
> Please add a new config option for this code instead of always building
> it when swiotlb is enabled.
>
> > +static int swiotlb_init_io_tlb_mem(struct io_tlb_mem *mem, phys_addr_t start,
> > +                                size_t size)
>
> Can you split the refactoring in swiotlb.c into one or more prep
> patches?
>
> > +static int rmem_swiotlb_device_init(struct reserved_mem *rmem,
> > +                                 struct device *dev)
> > +{
> > +     struct io_tlb_mem *mem = rmem->priv;
> > +     int ret;
> > +
> > +     if (dev->dma_io_tlb_mem)
> > +             return -EBUSY;
> > +
> > +     if (!mem) {
> > +             mem = kzalloc(sizeof(*mem), GFP_KERNEL);
> > +             if (!mem)
> > +                     return -ENOMEM;
>
> What is the calling convention here that allows for a NULL and non-NULL
> private data?

Since multiple devices can share the same pool, the private data,
io_tlb_mem struct, will be initialized by the first device attached to
it.
This is similar to rmem_dma_device_init() in kernel/dma/coherent.c.
I'll add a comment for it in next version.