netdev - Re: [PATCH v9 03/27] cxl: add capabilities field to cxl_dev_state and cxl

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Message-ID: <ad7ed7a4-4cfc-7e68-8d53-772efe542a96@amd.com>
Date: Fri, 3 Jan 2025 11:50:03 +0000
From: Alejandro Lucero Palau <alucerop@....com>
To: Jonathan Cameron <Jonathan.Cameron@...wei.com>
Cc: alejandro.lucero-palau@....com, linux-cxl@...r.kernel.org,
 netdev@...r.kernel.org, dan.j.williams@...el.com, edward.cree@....com,
 davem@...emloft.net, kuba@...nel.org, pabeni@...hat.com,
 edumazet@...gle.com, dave.jiang@...el.com
Subject: Re: [PATCH v9 03/27] cxl: add capabilities field to cxl_dev_state and
 cxl_port


On 1/3/25 10:50, Jonathan Cameron wrote:
> On Fri, 3 Jan 2025 07:20:48 +0000
> Alejandro Lucero Palau <alucerop@....com> wrote:
>
>> On 1/2/25 14:36, Jonathan Cameron wrote:
>>> On Mon, 30 Dec 2024 21:44:21 +0000
>>> <alejandro.lucero-palau@....com> wrote:
>>>   
>>>> From: Alejandro Lucero <alucerop@....com>
>>>>
>>>> Type2 devices have some Type3 functionalities as optional like an mbox
>>>> or an hdm decoder, and CXL core needs a way to know what an CXL accelerator
>>>> implements.
>>>>
>>>> Add a new field to cxl_dev_state for keeping device capabilities as
>>>> discovered during initialization. Add same field to cxl_port as registers
>>>> discovery is also used during port initialization.
>>>>
>>>> Signed-off-by: Alejandro Lucero <alucerop@....com>
>>>> Reviewed-by: Ben Cheatham <benjamin.cheatham@....com>
>>>> Reviewed-by: Fan Ni <fan.ni@...sung.com>
>>> Comment in thread on v8.  I don't see a reason to have any specific
>>> bitmap length - just use a final entry in the enum without a value set
>>> to let us know how long it actually is.
>>
>> I could do this but it implies to clear/zeroing the bitmaps with the
>> final entry value and to mask bitmaps with that when comparing them.
> Yes but that is automatic if you use the bitmap functions throughout.
>

Oh, that is true.


>> I tried to avoid the masking, and it led to that use of sizeof and then
>> CXL_MAX_CAPS=64.
> Don't avoid it. You are creating maintenance pain for a bit of unnecessary
> micro optimization.  Just make sure to treat this bitmap as a bitmap
> in all paths and there will be not reason for a reviewer to ever have
> to care what this value is and whether enough bits are zero etc.


I'm afraid I have been in the wrong path regarding the use of the bitmap 
API.

I even initially implemented my own bitmap_subset because I did not 
notice there was one already there, and I think that was the starting 
point of this chain of bad decisions. Because I did implement it poorly 
it led to keep with the wrong assumptions when I was told to use the 
existing one.


Happy to have you pointing this out. I'll fix it.


Thank you!



> Jonathan
>
>
>
>
>>
>>> Using the bit / bitmap functions should work fine without constraining
>>> that to any particular value - also allowing for greater than 64 entries
>>> with no need to fix up call sites etc.
>>>
>>>   
>>>> diff --git a/drivers/cxl/core/regs.c b/drivers/cxl/core/regs.c
>>>> index 59cb35b40c7e..144ae9eb6253 100644
>>>> --- a/drivers/cxl/core/regs.c
>>>> +++ b/drivers/cxl/core/regs.c
>>>> @@ -4,6 +4,7 @@
>>>> +enum cxl_dev_cap {
>>>> +	/* capabilities from Component Registers */
>>>> +	CXL_DEV_CAP_RAS,
>>>> +	CXL_DEV_CAP_HDM,
>>>> +	/* capabilities from Device Registers */
>>>> +	CXL_DEV_CAP_DEV_STATUS,
>>>> +	CXL_DEV_CAP_MAILBOX_PRIMARY,
>>>> +	CXL_DEV_CAP_MEMDEV,
>>>> +	CXL_MAX_CAPS = 64
>>> As in v8. I'm not seeing any reason for this.  If you need
>>> a bitmap to be a particular number of unsigned longs, then that
>>> code should be fixed. (only exception being compile time constant
>>> bitmaps where this is tricky to do!)
>>>
>>> Obviously I replied with that to v8 after you posted this
>>> so time machines aside no way you could have acted on it yet.
>>>
>>>
>>> Jonathan
>>>   
>>>> +};
>>>> +
>>>>    struct cxl_dev_state;
>>>>    struct device;
>>>>