linux-kernel - Re: [PATCH v6 04/15] swiotlb: Add restricted DMA pool initialization

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Date:   Wed, 12 May 2021 00:42:28 +0800
From:   Claire Chang <tientzu@...omium.org>
To:     Christoph Hellwig <hch@....de>
Cc:     Rob Herring <robh+dt@...nel.org>, mpe@...erman.id.au,
        Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
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Subject: Re: [PATCH v6 04/15] swiotlb: Add restricted DMA pool initialization

On Mon, May 10, 2021 at 11:03 PM Christoph Hellwig <hch@....de> wrote:
>
> > +#ifdef CONFIG_DMA_RESTRICTED_POOL
> > +#include <linux/io.h>
> > +#include <linux/of.h>
> > +#include <linux/of_fdt.h>
> > +#include <linux/of_reserved_mem.h>
> > +#include <linux/slab.h>
> > +#endif
>
> I don't think any of this belongs into swiotlb.c.  Marking
> swiotlb_init_io_tlb_mem non-static and having all this code in a separate
> file is probably a better idea.

Will do in the next version.

>
> > +#ifdef CONFIG_DMA_RESTRICTED_POOL
> > +static int rmem_swiotlb_device_init(struct reserved_mem *rmem,
> > +                                 struct device *dev)
> > +{
> > +     struct io_tlb_mem *mem = rmem->priv;
> > +     unsigned long nslabs = rmem->size >> IO_TLB_SHIFT;
> > +
> > +     if (dev->dma_io_tlb_mem)
> > +             return 0;
> > +
> > +     /* 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 not the normal kernel comment style.

Will fix this in the next version.

>
> > +#ifdef CONFIG_ARM
> > +             if (!PageHighMem(pfn_to_page(PHYS_PFN(rmem->base)))) {
> > +                     kfree(mem);
> > +                     return -EINVAL;
> > +             }
> > +#endif /* CONFIG_ARM */
>
> And this is weird.  Why would ARM have such a restriction?  And if we have
> such rstrictions it absolutely belongs into an arch helper.

Now I think the CONFIG_ARM can just be removed?
The goal here is to make sure we're using linear map and can safely
use phys_to_dma/dma_to_phys.

>
> > +             swiotlb_init_io_tlb_mem(mem, rmem->base, nslabs, false);
> > +
> > +             rmem->priv = mem;
> > +
> > +#ifdef CONFIG_DEBUG_FS
> > +             if (!debugfs_dir)
> > +                     debugfs_dir = debugfs_create_dir("swiotlb", NULL);
> > +
> > +             swiotlb_create_debugfs(mem, rmem->name, debugfs_dir);
>
> Doesn't the debugfs_create_dir belong into swiotlb_create_debugfs?  Also
> please use IS_ENABLEd or a stub to avoid ifdefs like this.

Will move it into swiotlb_create_debugfs and use IS_ENABLED in the next version.