linux-kernel - Re: [PATCH 5/6] dmaengine: Add Broadcom SBA RAID driver

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Message-ID: <CAALAos8W13Xcxw+VpkvJWNNEiO6_6Hri0EheASHqOzCKagDYzA@mail.gmail.com>
Date:   Tue, 7 Feb 2017 11:32:23 +0530
From:   Anup Patel <anup.patel@...adcom.com>
To:     Vinod Koul <vinod.koul@...el.com>
Cc:     Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "David S . Miller" <davem@...emloft.net>,
        Jassi Brar <jassisinghbrar@...il.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Ray Jui <rjui@...adcom.com>,
        Scott Branden <sbranden@...adcom.com>,
        Jon Mason <jonmason@...adcom.com>,
        Rob Rice <rob.rice@...adcom.com>,
        BCM Kernel Feedback <bcm-kernel-feedback-list@...adcom.com>,
        dmaengine@...r.kernel.org,
        Device Tree <devicetree@...r.kernel.org>,
        Linux ARM Kernel <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel <linux-kernel@...r.kernel.org>,
        linux-crypto@...r.kernel.org,
        linux-raid <linux-raid@...r.kernel.org>
Subject: Re: [PATCH 5/6] dmaengine: Add Broadcom SBA RAID driver

On Mon, Feb 6, 2017 at 10:24 PM, Vinod Koul <vinod.koul@...el.com> wrote:
> On Mon, Feb 06, 2017 at 05:31:15PM +0530, Anup Patel wrote:
>
>> >> +
>> >> +/* SBA C_MDATA helper macros */
>> >> +#define SBA_C_MDATA_LOAD_VAL(__bnum0)                ((__bnum0) & 0x3)
>> >> +#define SBA_C_MDATA_WRITE_VAL(__bnum0)               ((__bnum0) & 0x3)
>> >> +#define SBA_C_MDATA_XOR_VAL(__bnum1, __bnum0)                        \
>> >> +                     ({      u32 __v = ((__bnum0) & 0x3);    \
>> >> +                             __v |= ((__bnum1) & 0x3) << 2;  \
>> >> +                             __v;                            \
>> >> +                     })
>> >> +#define SBA_C_MDATA_PQ_VAL(__dnum, __bnum1, __bnum0)         \
>> >> +                     ({      u32 __v = ((__bnum0) & 0x3);    \
>> >> +                             __v |= ((__bnum1) & 0x3) << 2;  \
>> >> +                             __v |= ((__dnum) & 0x1f) << 5;  \
>> >> +                             __v;                            \
>> >> +                     })
>> >
>> > ah why are we usig complex macros, why can't these be simple functions..
>>
>> "static inline functions" seemed too complicated here because most of
>> these macros are two lines of c-code.
>
> and thats where I have an issue with this. Macros for simple things is fine
> but not for couple of line of logic!
>
>>
>> Do you still insist on using "static inline functions"?
>
> Yes

Sure, will use "static inline functions" instead these macros.

>
>>
>> >
>> >> +#define SBA_C_MDATA_LS(__c_mdata_val)        ((__c_mdata_val) & 0xff)
>> >> +#define SBA_C_MDATA_MS(__c_mdata_val)        (((__c_mdata_val) >> 8) & 0x3)
>> >> +
>> >> +/* Driver helper macros */
>> >> +#define to_sba_request(tx)           \
>> >> +     container_of(tx, struct sba_request, tx)
>> >> +#define to_sba_device(dchan)         \
>> >> +     container_of(dchan, struct sba_device, dma_chan)
>> >> +
>> >> +enum sba_request_state {
>> >> +     SBA_REQUEST_STATE_FREE = 1,
>> >> +     SBA_REQUEST_STATE_ALLOCED = 2,
>> >> +     SBA_REQUEST_STATE_PENDING = 3,
>> >> +     SBA_REQUEST_STATE_ACTIVE = 4,
>> >> +     SBA_REQUEST_STATE_COMPLETED = 5,
>> >> +     SBA_REQUEST_STATE_ABORTED = 6,
>> >
>> > whats up with a very funny indentation setting, we use 8 chars.
>> >
>> > Please re-read the Documentation/process/coding-style.rst
>>
>> I have double checked this enum. The indentation is fine
>> and as-per coding style. Am I missing anything else?
>
> Somehow the initial indent doesnt seem to be 8 chars to me.
>
>> >> +static enum dma_status sba_tx_status(struct dma_chan *dchan,
>> >> +                                  dma_cookie_t cookie,
>> >> +                                  struct dma_tx_state *txstate)
>> >> +{
>> >> +     int mchan_idx;
>> >> +     enum dma_status ret;
>> >> +     struct sba_device *sba = to_sba_device(dchan);
>> >> +
>> >> +     ret = dma_cookie_status(dchan, cookie, txstate);
>> >> +     if (ret == DMA_COMPLETE)
>> >> +             return ret;
>> >> +
>> >> +     for (mchan_idx = 0; mchan_idx < sba->mchans_count; mchan_idx++)
>> >> +             mbox_client_peek_data(sba->mchans[mchan_idx]);
>> >
>> > what is this achieving?
>>
>> The mbox_client_peek_data() is a hint to mailbox controller driver
>> to check for available messages.
>>
>> This gives good performance improvement when some DMA client
>> code is polling using tx_status() callback.
>
> Then why do it before and then check status.

If there was a work completed when mbox_client_peek_data()
is called then sba_receive_message() will be called immediately
by mailbox controller driver.

We are doing dma_cookie_complete() in sba_receive_message()
so if mbox_client_peek_data() is called before dma_cookie_status()
then dma_cookie_status() will see correct state of cookie.

Also, I explored virt-dma APIs for BCM-SBA-RAID driver. The virt-dma
implements tasklet based bottom-half for each virt-dma-channel. This
bottom-half is not required for BCM-FS4-RAID driver because its a
mailbox client driver and the mailbox controller driver already implements
bottom-half for each mailbox channel.

If we still go ahead and use virt-dma in BCM-FS4-RAID driver then we
will have two bottom-halfs in-action one in mailbox controller driver and
another in BCM-FS4-RAID driver which in-turn will add bottom-half
scheduling overhead thereby reducing performance of BCM-FS4-RAID
driver.

Regards,
Anup