Re: [PATCH v4 1/2] cxl/region: Find free cxl decoder by device_for_each_child()

[Zijun Hu]

On 2024/9/11 00:01, Dan Williams wrote:
> Zijun Hu wrote:
> [..]
>>
>> do not known how you achieve it, perhaps, it is not simpler than
>> my below solution:
>>
>> finding a free switch cxl decoder with minimal ID
>> https://lore.kernel.org/all/20240905-fix_cxld-v2-1-51a520a709e4@xxxxxxxxxxx/
>>
>> which has simple logic and also does not have any limitation related
>> to add/allocate/de-allocate a decoder.
>>
>> i am curious why not to consider this solution ?
> 
> Because it leaves region shutdown ordering bug in place.
> 
>>> 2/ search for decoders in their added order: done, device_find_child()
>>> guarantees this, although it is not obvious without reading the internals
>>> of device_add().
>>>
>>> 3/ regions are de-allocated from decoders in reverse decoder id order.
>>> This is not enforced, in fact it is impossible to enforce. Consider that
>>> any memory device can be removed at any time and may not be removed in
>>> the order in which the device allocated switch decoders in the topology.
>>>
>>
>> sorry, don't understand, could you take a example ?
>>
>> IMO, the simple change in question will always get a free decoder with
>> the minimal ID once 1/ is ensured regardless of de-allocation approach.
> 
> No, you are missing the fact that CXL hardware requires that decoders
> cannot be sparsely allocated. They must be allocated consecutively and
> in increasing address order.
> 
> Imagine a scenario with a switch port with three decoders,
> decoder{A,B,C} allocated to 3 respective regions region{A,B,C}.
> 
> If regionB is destroyed due to device removal that does not make
> decoderB free to be reallocated in hardware. The destruction of regionB
> requires regionC to be torn down first. As it stands the driver does not
> force regionC to shutdown and it falsely clears @decoderB->region making
> it appear free prematurely.
> 
> So, while regionB would be the next decoder to allocate after regionC is
> torn down, it is not a free decoder while decoderC and regionC have not been
> reclaimed.

understood it due to your detailed explanation. thank you Dan.