linux-kernel - Re: [PATCH 13/18] ide: use ->tf_load in SELECT

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Date:	Tue, 17 Feb 2009 04:04:11 +0300
From:	Sergei Shtylyov <sshtylyov@...mvista.com>
To:	Bartlomiej Zolnierkiewicz <bzolnier@...il.com>
Cc:	linux-ide@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 13/18] ide: use ->tf_load in SELECT_DRIVE()

Hello.

Bartlomiej Zolnierkiewicz wrote:
> On Monday 16 February 2009, Sergei Shtylyov wrote:
>   
>> Hello, I wrote:
>>
>>     
>>>>> There should be no functional changes caused by this patch.
>>>>> Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@...il.com>
>>>>> Index: b/drivers/ide/ide-iops.c
>>>>> ===================================================================
>>>>> --- a/drivers/ide/ide-iops.c
>>>>> +++ b/drivers/ide/ide-iops.c
>>>>> @@ -88,11 +88,15 @@ void SELECT_DRIVE (ide_drive_t *drive)
>>>>>  {
>>>>>      ide_hwif_t *hwif = drive->hwif;
>>>>>      const struct ide_port_ops *port_ops = hwif->port_ops;
>>>>> +    ide_task_t task;
>>>>>  
>>>>>      if (port_ops && port_ops->selectproc)
>>>>>          port_ops->selectproc(drive);
>>>>>  
>>>>> -    hwif->OUTB(drive->select.all, hwif->io_ports.device_addr);
>>>>> +    memset(&task, 0, sizeof(task));
>>>>> +    task.tf_flags = IDE_TFLAG_OUT_DEVICE;
>>>>> +
>>>>> +    drive->hwif->tf_load(drive, &task);
>>>>>           
>>>>    This actually doesn't seem like a bright idea to me, considering 
>>>> that this gets called when starting every request. How will you look 
>>>> at me adding the transport method for writing this register? :-)
>>>>         
>
> Please check profiles first -- it might not be worth it. [1]
>
>   
>>> Convert SELECT_DRIVE() to use ->tf_load instead of ->OUTB.
>>>
>>>   OTOH, adding such a "backdoor" to the taskfile doesn't seem very 
>>> consistent... well, I'm not excited about the whole idea conversion to 
>>> tf_{load|read}() -- it's not clear what exactly this bought us.
>>>       
>
> This was explained some months ago already, so just to recall -- it was
> a part of a bigger work removing duplicated code and allowing abstraction
> of the ATA logic.
>
> Anyway this is not set in a stone so if you have proposal of a better
> approach please come forward with it.
>   

   Er... I think that the previous IN()/OUT() methods were better. Note 
that we ended up using the local version of them in the dafault 
ide_tf_{load}read}() anyway -- as Alan has pointed out it might be worth 
splitting those into I/O and memory space versions... although given 
general slowness of the I/O accesses, this is probably not going to win 
much speed-wise.

>>    We at least could have saved on memset() -- tf_load() method ignores 
>> fields other than tf_flags anyway...
>>     
>
> Unless it is huge performance win (unlikely) this is not a good idea as it would be a maintainance nightmare.
>   
> ->tf_load does only use cmd->tf_flags today but it might change one day
> and nobody will remember to audit all users that they pass a valid cmd...
>   

   It's just quite unbearable to see (especially for a long time 
assembly coder) how a single register write is turning into *that*.
So, it still seems worth risking... :-)

> Thanks,
> Bart
>   

MBR, Sergei


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