netdev - Re: [PATCH v4 1/2] cxl/region: Find free cxl decoder by device_for_each

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Message-ID: <1b1cf618-b62f-43ab-b2dd-29398e33a43d@icloud.com>
Date: Wed, 11 Sep 2024 19:52:09 +0800
From: Zijun Hu <zijun_hu@...oud.com>
To: Dan Williams <dan.j.williams@...el.com>,
 quic_zijuhu <quic_zijuhu@...cinc.com>, Ira Weiny <ira.weiny@...el.com>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Davidlohr Bueso <dave@...olabs.net>,
 Jonathan Cameron <jonathan.cameron@...wei.com>,
 Dave Jiang <dave.jiang@...el.com>,
 Alison Schofield <alison.schofield@...el.com>,
 Vishal Verma <vishal.l.verma@...el.com>, Timur Tabi <timur@...nel.org>,
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 linux-cxl@...r.kernel.org, linux-kernel@...r.kernel.org,
 netdev@...r.kernel.org
Subject: Re: [PATCH v4 1/2] cxl/region: Find free cxl decoder by
 device_for_each_child()

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@quicinc.com/
>>
>> 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.