linux-kernel - Re: [PATCH 18/21] scsi: sd: Support reading atomic properties from block limits VPD

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Date:   Mon, 2 Oct 2023 12:27:52 +0100
From:   John Garry <john.g.garry@...cle.com>
To:     Bart Van Assche <bvanassche@....org>, axboe@...nel.dk,
        kbusch@...nel.org, hch@....de, sagi@...mberg.me,
        jejb@...ux.ibm.com, martin.petersen@...cle.com, djwong@...nel.org,
        viro@...iv.linux.org.uk, brauner@...nel.org,
        chandan.babu@...cle.com, dchinner@...hat.com
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-nvme@...ts.infradead.org, linux-xfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, tytso@....edu, jbongio@...gle.com,
        linux-api@...r.kernel.org
Subject: Re: [PATCH 18/21] scsi: sd: Support reading atomic properties from
 block limits VPD

On 29/09/2023 18:54, Bart Van Assche wrote:
> On 9/29/23 03:27, John Garry wrote:
>> +static void sd_config_atomic(struct scsi_disk *sdkp)
>> +{
>> +    unsigned int logical_block_size = sdkp->device->sector_size;
>> +    struct request_queue *q = sdkp->disk->queue;
>> +
>> +    if (sdkp->max_atomic) {
> 
> Please use the "return early" style here to keep the indentation
> level in this function low.

ok, fine.

> 
>> +        unsigned int max_atomic = max_t(unsigned int,
>> +            rounddown_pow_of_two(sdkp->max_atomic),
>> +            rounddown_pow_of_two(sdkp->max_atomic_with_boundary));
>> +        unsigned int unit_min = sdkp->atomic_granularity ?
>> +            rounddown_pow_of_two(sdkp->atomic_granularity) :
>> +            physical_block_size_sectors;
>> +        unsigned int unit_max = max_atomic;
>> +
>> +        if (sdkp->max_atomic_boundary)
>> +            unit_max = min_t(unsigned int, unit_max,
>> +                rounddown_pow_of_two(sdkp->max_atomic_boundary));
> 
> Why does "rounddown_pow_of_two()" occur in the above code?

I assume that you are talking about all the code above to calculate 
atomic write values for the device.

The reason is that atomic write unit min and max are always a power-of-2 
- see rules described earlier - as so that we why we rounddown to a 
power-of-2.

Thanks,
John