Date:   Mon, 11 Feb 2019 10:56:18 +0200
From:   Adrian Hunter <adrian.hunter@...el.com>
To:     Alamy Liu <alamy.liu@...il.com>,
        "Asutosh Das (asd)" <asutoshd@...eaurora.org>,
        Ritesh Harjani <riteshh@...eaurora.org>
Cc:     Ulf Hansson <ulf.hansson@...aro.org>,
        "open list:MULTIMEDIA CARD (MMC), SECURE DIGITAL (SD) AND..." 
        <linux-mmc@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] mmc: cqhci: replace NUM_SLOTS with cq_host->num_slots

On 8/02/19 8:41 PM, Alamy Liu wrote:
> The IP I'm using has fixed 32 slots for CQ once CQE is enabled in RTL.
> There is no other RO register to say the slot number & which slot that
> DCMD would use.
> 
> I agree that most host controller would have 32 slots for CQ, and use
> the last one for DCMD if enabled (easier design & save gates
> count/size). (15th for DCMD if there were 16 slots in CQ, ...etc). It
> also doesn't make sense to use a slot in the middle of slots for DCMD,
> Both the code (both S.W. & H.W.) would become more complicated.
> Although I don't see it is defined somewhere, I believe that every
> designer would follow.
> (H.W.: increasing gate count -> increasing die size -> increase $;
> complicated design -> higher chance to have bug/error).
> (H.W.: SDHC cq_depth: longer cq_depth -> more memory -> more $. The
> decision would be the trade off between $ & performance. Also, the
> SDHC is not necessary to be used in Cortex-A/Linux. Thus, 16/8
> cq_depth is possible, although I don't know if there is one existed
> out there)
> 
> Another thinking: If all cq_depth is fixed to 32, the mmc driver core
> doesn't even have to detect the cq_depth and eventually choose the
> small one between HC & eMMC (queue.c::mmc_init_queue)

The cqe_qdepth changes depending on whether DCMD is supported.  That is the
kind of detail that belongs to the CQHCI driver not the mmc core.  For
example, theoretically there could be more than one CQE driver
implementation, but it is also nicer to keep a separation between CQHCI and
mmc core.

> 
> Best Regards
> 
> 
> On Fri, Feb 8, 2019 at 2:34 AM Asutosh Das (asd)
> <asutoshd@...eaurora.org> wrote:
>>
>> On 2/8/2019 8:07 AM, Ritesh Harjani wrote:
>>> Hi Alamy,
>>>
>>> On 2/8/2019 1:00 AM, Alamy Liu wrote:
>>>> It says in B.2.1 in the JESD84-B51.pdf (I don't have JESD84-B51A.pdf):
>>>>
>>>>     /The TDL is located in a memory location known to the CQE, and is
>>>>     comprised of up to 32 fixed-size slots. Each slot is comprised of
>>>>     one Task Descriptor and one Transfer Descriptor./
>>>>
>>>>
>>>> So if the IP has 16 slots, it should still meet the specification.
>>>> Then the configuration could be moved to DTS, maybe:
>>>>
>>>>     cqhci-slotnum = <16>;
>>>>
>>>>     cqhci-dcmd-slotno = <15>;
>>>>
>>> Does your IP defines this as 16 slots & DCMD slot to be 15?
>>> Because if there is a deviation then IP may also define num_slots by
>>> using some reserved registers.
>>> Also in JESD84-B51.pdf, specific slot no. is used extensively for
>>> defining policies(like in case of DCMD & CQCFG), so in that case
>>> defining via DT may not be that helpful.
>>> Unless there is no other way in your IP to determine the num_slots
>>> except going via DT?
>>>
>>> Let others also provide an opinion here.
>>>
>>> Regards
>>> Ritesh
>>>
>>>
>>>>
>>>> Please comment.
>>>>
>>>> Regards,
>>>> Alamy
>>>>
>>>>
>>>>
>>>> On Thu, Jan 31, 2019 at 1:34 AM Adrian Hunter <adrian.hunter@...el.com
>>>> <mailto:adrian.hunter@...el.com>> wrote:
>>>>
>>>>     On 14/01/19 9:17 PM, Alamy Liu wrote:
>>>>     > Prevent to use fixed value (NUM_SLOTS) after it had been determined
>>>>     > and saved in a variable (cq_host->num_slots).
>>>>
>>>>     num_slots is always 32 (i.e. NUM_SLOTS) so why not go the other
>>>>     way and get
>>>>     rid of num_slots and just use NUM_SLOTS?
>>>>
>>>>     >
>>>>     > Signed-off-by: Alamy Liu <alamy.liu@...il.com
>>>>     <mailto:alamy.liu@...il.com>>
>>>>     > ---
>>>>     >  drivers/mmc/host/cqhci.c | 2 +-
>>>>     >  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>     >
>>>>     > diff --git a/drivers/mmc/host/cqhci.c b/drivers/mmc/host/cqhci.c
>>>>     > index 159270e947..26d63594b7 100644
>>>>     > --- a/drivers/mmc/host/cqhci.c
>>>>     > +++ b/drivers/mmc/host/cqhci.c
>>>>     > @@ -699,7 +699,7 @@ static void cqhci_error_irq(struct mmc_host
>>>>     *mmc, u32 status, int cmd_error,
>>>>     >                * The only way to guarantee forward progress is
>>>>     to mark at
>>>>     >                * least one task in error, so if none is
>>>>     indicated, pick one.
>>>>     >                */
>>>>     > -             for (tag = 0; tag < NUM_SLOTS; tag++) {
>>>>     > +             for (tag = 0; tag < cq_host->num_slots; tag++) {
>>>>     >                       slot = &cq_host->slot[tag];
>>>>     >                       if (!slot->mrq)
>>>>     >                               continue;
>>>>     >
>>>>
>> Is it worth exploring to tie up the TDL memory allocations with the
>> queue-depth? Because the queue-depth may vary with vendor; in most host
>> controllers the slot size is 32. And since memory allocations are done
>> on the basis of host slot size there's unused slots in the case of card
>> advertising less than 32 queue-depth.
>> The tricky part would be the DCMD handling though.
>>
>> In the IP in question, what slot is assigned to DCMD?
>>
>>
>> --
>> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a
>> Linux Foundation Collaborative Project
> 

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