linux-kernel - Re: [RFC PATCH v3 6/6] of: Add plumbing for restricted DMA pool

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Date:   Thu, 14 Jan 2021 10:52:44 -0800
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Claire Chang <tientzu@...omium.org>
Cc:     Rob Herring <robh+dt@...nel.org>, mpe@...erman.id.au,
        benh@...nel.crashing.org, paulus@...ba.org,
        "list@....net:IOMMU DRIVERS" <iommu@...ts.linux-foundation.org>,
        Joerg Roedel <joro@...tes.org>, joro@...tes.org,
        will@...nel.org, Frank Rowand <frowand.list@...il.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        boris.ostrovsky@...cle.com, jgross@...e.com,
        sstabellini@...nel.org, Christoph Hellwig <hch@....de>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>, grant.likely@....com,
        xypron.glpk@....de, Thierry Reding <treding@...dia.com>,
        mingo@...nel.org, bauerman@...ux.ibm.com, peterz@...radead.org,
        Greg KH <gregkh@...uxfoundation.org>,
        Saravana Kannan <saravanak@...gle.com>,
        rafael.j.wysocki@...el.com, heikki.krogerus@...ux.intel.com,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        rdunlap@...radead.org, dan.j.williams@...el.com,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        linux-devicetree <devicetree@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        linuxppc-dev@...ts.ozlabs.org,
        "list@....net:IOMMU DRIVERS" <iommu@...ts.linux-foundation.org>,
        Joerg Roedel <joro@...tes.org>,
        iommu@...ts.linux-foundation.org, xen-devel@...ts.xenproject.org,
        Tomasz Figa <tfiga@...omium.org>,
        Nicolas Boichat <drinkcat@...omium.org>
Subject: Re: [RFC PATCH v3 6/6] of: Add plumbing for restricted DMA pool

On 1/14/21 1:08 AM, Claire Chang wrote:
> On Wed, Jan 13, 2021 at 7:48 AM Florian Fainelli <f.fainelli@...il.com> wrote:
>>
>> On 1/5/21 7:41 PM, Claire Chang wrote:
>>> If a device is not behind an IOMMU, we look up the device node and set
>>> up the restricted DMA when the restricted-dma-pool is presented.
>>>
>>> Signed-off-by: Claire Chang <tientzu@...omium.org>
>>> ---
>>
>> [snip]
>>
>>> +int of_dma_set_restricted_buffer(struct device *dev)
>>> +{
>>> +     struct device_node *node;
>>> +     int count, i;
>>> +
>>> +     if (!dev->of_node)
>>> +             return 0;
>>> +
>>> +     count = of_property_count_elems_of_size(dev->of_node, "memory-region",
>>> +                                             sizeof(phandle));
>>
>> You could have an early check for count < 0, along with an error
>> message, if that is deemed useful.
>>
>>> +     for (i = 0; i < count; i++) {
>>> +             node = of_parse_phandle(dev->of_node, "memory-region", i);
>>> +             if (of_device_is_compatible(node, "restricted-dma-pool"))
>>
>> And you may want to add here an of_device_is_available(node). A platform
>> that provides the Device Tree firmware and try to support multiple
>> different SoCs may try to determine if an IOMMU is present, and if it
>> is, it could be marking the restriced-dma-pool region with a 'status =
>> "disabled"' property, or any variant of that scheme.
> 
> This function is called only when there is no IOMMU present (check in
> drivers/of/device.c). I can still add of_device_is_available(node)
> here if you think it's helpful.

I believe it is, since boot loader can have a shared Device Tree blob
skeleton and do various adaptations based on the chip (that's what we
do) and adding a status property is much simpler than insertion new
nodes are run time.

> 
>>
>>> +                     return of_reserved_mem_device_init_by_idx(
>>> +                             dev, dev->of_node, i);
>>
>> This does not seem to be supporting more than one memory region, did not
>> you want something like instead:
>>
>>                 ret = of_reserved_mem_device_init_by_idx(...);
>>                 if (ret)
>>                         return ret;
>>
> 
> Yes. This implement only supports one restriced-dma-pool memory region
> with the assumption that there is only one memory region with the
> compatible string, restricted-dma-pool, in the dts. IIUC, it's similar
> to shared-dma-pool.

Then if here is such a known limitation it should be both documented and
enforced here, you shouldn ot be iterating over all of the phandles that
you find, stop at the first one and issue a warning if count > 1?
-- 
Florian