[PATCH v2] bus: mhi: host: Allocate entire MHI control config once

[Baochen Qiang]

On 5/16/2025 11:09 PM, Jeff Hugo wrote:
> On 5/12/2025 7:43 PM, Baochen Qiang wrote:
>>
>>
>> On 5/13/2025 2:31 AM, Jeff Hugo wrote:
>>> On 4/27/2025 7:57 PM, Baochen Qiang wrote:
>>>>
>>>>
>>>> On 4/26/2025 1:04 AM, Jeff Hugo wrote:
>>>>> From: Pranjal Ramajor Asha Kanojiya <quic_pkanojiy at quicinc.com>
>>>>>    int mhi_init_mmio(struct mhi_controller *mhi_cntrl)
>>>>> @@ -475,6 +497,7 @@ int mhi_init_mmio(struct mhi_controller *mhi_cntrl)
>>>>>        struct mhi_event *mhi_event;
>>>>>        void __iomem *base = mhi_cntrl->regs;
>>>>>        struct device *dev = &mhi_cntrl->mhi_dev->dev;
>>>>> +    dma_addr_t mhi_ctrl_limit = mhi_cntrl->ctrl_config_dma + mhi_cntrl-
>>>>>> ctrl_config_size - 1;
>>>>
>>>> but the range [ctrl_config_dma, mhi_ctrl_limit] could still be possbile to cross the 4GB
>>>> boundary, no?
>>>
>>> Its possible, yes.  From a practical standpoint, that seems to be unlikely as the control
>>> area is not terribly large on supported platforms.
>>
>> although unlikely, we need to handle it, right?
>>
>> IMO a possible solution is that you allocate (2 * ctrl_config_size  - 1) bytes and take
>> either the former half or the latter half, depending on which one does not cross the
>> boundary.
>>
> 
> Ick. That is going to double the memory consumption for the lifetime of the device, and
> these are dma_alloc_coherent() allocations which usually pulls from a special pool, making
> it a limited resource in my view. qaic has usecases for 128 devices in a system, meaning
> 128 allocations are now doubled. That feels excessive.
> 
> Trying to allocate again may introduce fragmentation, which is also something I'd like to
> avoid.
> 
> I'm working to get the spec updated to remove this restriction, which would then mean only
> "legacy" devices are a concern. As far as I know, the number of MHI devices that actually
> require this handling are low compared to the set of all MHI devices.  Therefore I am
> hoping for a balance.

Really not a fan of such balance, especially at a price of breaking legacy devices.

> 
> -Jeff