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

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Date:	Tue, 17 Feb 2009 18:32:02 +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()

Bartlomiej Zolnierkiewicz 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 

> During ide_tf_{load,read}() addition I was a bit too optimistic about
> the possibility of the quick io{read,write}* conversion later...

>>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.

> Maybe it would be worth to add ->tf_{inb,outb} to struct ide_tp_ops
> and convert default tp_ops to use them...  OTOH we should reinvestigate
> the io{read,write}*() way first (maybe things have improved there)...

    Yes, let's not be hasty here...

>>>>   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... :-)

> I see your point here.  If SELECT_DRIVE() is performance sensitive we
> may just add another struct ide_tp_ops method for it...

    Or we may finally teach selectproc() to also do that, turning it into 
analog of libata's dev_select() method.

> Thanks,
> Bart

MBR, Sergei
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