Re: [RFC PATCH v3 2/6] swiotlb: Add restricted DMA pool

[Robin Murphy <robin.murphy@arm.com>]

On 2021-01-13 17:43, Florian Fainelli wrote:
> On 1/13/21 7:27 AM, Robin Murphy wrote:
>> On 2021-01-13 13:59, Nicolas Saenz Julienne wrote:
>>> Hi All,
>>>
>>> On Tue, 2021-01-12 at 16:03 -0800, Florian Fainelli wrote:
>>>> On 1/5/21 7:41 PM, Claire Chang wrote:
>>>>> Add the initialization function to create restricted DMA pools from
>>>>> matching reserved-memory nodes in the device tree.
>>>>>
>>>>> Signed-off-by: Claire Chang <tientzu@chromium.org>
>>>>> ---
>>>>>    include/linux/device.h  |   4 ++
>>>>>    include/linux/swiotlb.h |   7 +-
>>>>>    kernel/dma/Kconfig      |   1 +
>>>>>    kernel/dma/swiotlb.c    | 144
>>>>> ++++++++++++++++++++++++++++++++++------
>>>>>    4 files changed, 131 insertions(+), 25 deletions(-)
>>>>>
>>>>> diff --git a/include/linux/device.h b/include/linux/device.h
>>>>> index 89bb8b84173e..ca6f71ec8871 100644
>>>>> --- a/include/linux/device.h
>>>>> +++ b/include/linux/device.h
>>>>> @@ -413,6 +413,7 @@ struct dev_links_info {
>>>>>     * @dma_pools:    Dma pools (if dma'ble device).
>>>>>     * @dma_mem:    Internal for coherent mem override.
>>>>>     * @cma_area:    Contiguous memory area for dma allocations
>>>>> + * @dma_io_tlb_mem: Internal for swiotlb io_tlb_mem override.
>>>>>     * @archdata:    For arch-specific additions.
>>>>>     * @of_node:    Associated device tree node.
>>>>>     * @fwnode:    Associated device node supplied by platform firmware.
>>>>> @@ -515,6 +516,9 @@ struct device {
>>>>>    #ifdef CONFIG_DMA_CMA
>>>>>        struct cma *cma_area;        /* contiguous memory area for dma
>>>>>                           allocations */
>>>>> +#endif
>>>>> +#ifdef CONFIG_SWIOTLB
>>>>> +    struct io_tlb_mem    *dma_io_tlb_mem;
>>>>>    #endif
>>>>>        /* arch specific additions */
>>>>>        struct dev_archdata    archdata;
>>>>> diff --git a/include/linux/swiotlb.h b/include/linux/swiotlb.h
>>>>> index dd8eb57cbb8f..a1bbd7788885 100644
>>>>> --- a/include/linux/swiotlb.h
>>>>> +++ b/include/linux/swiotlb.h
>>>>> @@ -76,12 +76,13 @@ extern enum swiotlb_force swiotlb_force;
>>>>>     *
>>>>>     * @start:    The start address of the swiotlb memory pool. Used
>>>>> to do a quick
>>>>>     *        range check to see if the memory was in fact allocated
>>>>> by this
>>>>> - *        API.
>>>>> + *        API. For restricted DMA pool, this is device tree
>>>>> adjustable.
>>>>
>>>> Maybe write it as this is "firmware adjustable" such that when/if ACPI
>>>> needs something like this, the description does not need updating.
>>
>> TBH I really don't think this needs calling out at all. Even in the
>> regular case, the details of exactly how and where the pool is allocated
>> are beyond the scope of this code - architectures already have several
>> ways to control that and make their own decisions.
>>
>>>>
>>>> [snip]
>>>>
>>>>> +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;
>>>>> +
>>>>> +        if (!memremap(rmem->base, rmem->size, MEMREMAP_WB)) {
>>>>
>>>> MEMREMAP_WB sounds appropriate as a default.
>>>
>>> As per the binding 'no-map' has to be disabled here. So AFAIU, this
>>> memory will
>>> be part of the linear mapping. Is this really needed then?
>>
>> More than that, I'd assume that we *have* to use the linear/direct map
>> address rather than anything that has any possibility of being a vmalloc
>> remap, otherwise we can no longer safely rely on
>> phys_to_dma/dma_to_phys, no?
> 
> I believe you are right, which means that if we want to make use of the
> restricted DMA pool on a 32-bit architecture (and we do, at least, I do)
> we should probably add some error checking/warning to ensure the
> restricted DMA pool falls within the linear map.

Oh, good point - I'm so used to 64-bit that I instinctively just blanked 
out the !PageHighMem() condition in try_ram_remap(). So maybe the 
original intent here *was* to effectively just implement that check, but 
if so it could still do with being a lot more explicit.

Cheers,
Robin.
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