linux-kernel - Re: [RFC 0/2] dw_mmc: add multislot support

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Date:   Fri, 20 Apr 2018 07:42:13 +0000
From:   Alexey Brodkin <Alexey.Brodkin@...opsys.com>
To:     "ulf.hansson@...aro.org" <ulf.hansson@...aro.org>
CC:     "jh80.chung@...sung.com" <jh80.chung@...sung.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mmc@...r.kernel.org" <linux-mmc@...r.kernel.org>,
        "krzk@...nel.org" <krzk@...nel.org>,
        "Eugeniy.Paltsev@...opsys.com" <Eugeniy.Paltsev@...opsys.com>,
        "linux-snps-arc@...ts.infradead.org" 
        <linux-snps-arc@...ts.infradead.org>,
        "kgene@...nel.org" <kgene@...nel.org>,
        "shawn.lin@...k-chips.com" <shawn.lin@...k-chips.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-samsung-soc@...r.kernel.org" 
        <linux-samsung-soc@...r.kernel.org>
Subject: Re: [RFC 0/2] dw_mmc: add multislot support

Hi Ulf,

On Fri, 2018-04-20 at 09:35 +0200, Ulf Hansson wrote:
> [...]
> 
> > 
> > 2. Add missing stuff to support multislot mode in DesignWare MMC driver.
> >  * Add missing slot switch to __dw_mci_start_request() function.
> >  * Refactor set_ios function:
> >    a) Calculate common clock which is
> >       suitable for all slots instead of directly use clock value
> >       provided by mmc core. We calculate common clock as the minimum
> >       among each used slot clocks. This clock is calculated in
> >       dw_mci_calc_common_clock() function which is called
> >       from set_ios()
> >    b) Disable clock only if no other slots are ON.
> >    c) Setup clock directly in set_ios() only if no other slots
> >       are ON. Otherwise adjust clock in __dw_mci_start_request()
> >       function before slot switch.
> >    d) Move timings and bus_width setup to separate funcions.
> >  * Use timing field in each slot structure instead of common field in
> >    host structure.
> >  * Add locks to serialize access to registers.
> 
> Sorry, but this is a hack to *try* to make multi-slot work and this
> isn't sufficient. There were good reasons to why the earlier
> non-working multi slot support was removed from dw_mmc.
> 
> Let me elaborate a bit for your understanding. The core uses a host
> lock (mmc_claim|release_host()) to serialize operations and commands,
> as to confirm to the SD/SDIO/(e)MMC specs. The above changes gives no
> guarantees for this. To make that work, we would need a "mmc bus lock"
> to be managed by the core.
> 
> However, inventing a "mmc bus lock" would lead to other problems
> related to I/O scheduling for upper layers - it simply breaks. For
> example, I/O requests for one card/slot can then starve I/O requests
> reaching another card/slot.

So are you saying that multi-slot support is a no go in general or it is only
applicable to DW MMC (I really doubt that's a case)?

BTW there're other controllers that seem to support multi-slot like Atmel etc.

-Alexey