Date:   Thu, 12 Dec 2019 03:09:32 +0000
From:   Peng Ma <peng.ma@....com>
To:     Russell King - ARM Linux admin <linux@...linux.org.uk>
CC:     "shawnguo@...nel.org" <shawnguo@...nel.org>,
        "s.hauer@...gutronix.de" <s.hauer@...gutronix.de>,
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        <linux-arm-kernel@...ts.infradead.org>,
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Subject: RE: [EXT] Re: [PATCH] i2c: imx: Defer probing if EDMA not available

Hello Russell,

Thanks very much for your strict guidance and comments.
I realized it is hard to us that we want to i2c used edma when edma
probe after i2c probe. I look forward to discussing with you as below, if you like.
Thanks.

You say I could do this:
"So, if you want to do this (and yes, I'd also encourage it to be
conditional on EDMA being built-in, as I2C is commonly used as a way
to get at RTCs, which are read before kernel modules can be loaded)
then you MUST move
i2c_imx_dma_request() before
i2c_add_numbered_adapter() to avoid the infinite loop."

Even if I do this, It's hard to avoid the infinite loop of i2c probe caused by EDMA(build-in) initialization failure.
I saw the function of_dma_request_slave_channel alloc chan as fallows:

ofdma = of_dma_find_controller(&dma_spec);

        if (ofdma) {
            chan = ofdma->of_dma_xlate(&dma_spec, ofdma);//dma probe successful when devices alloc dma slave channel
        } else {
            ret_no_channel = -EPROBE_DEFER; //dma not probe or probe failed when devices alloc dma slave channel
            chan = NULL;
        }   

Due to this case,we should make sure:
1. EDMA build-in
2. EDMA can probe successful

The first can realize, but I don't know how to check whether EDMA has been probed failed in i2c,
If we could check it, i2c will skip that loop.

Best Regards,
Peng
>-----Original Message-----
>From: Russell King - ARM Linux admin <linux@...linux.org.uk>
>Sent: 2019年12月11日 19:43
>To: Peng Ma <peng.ma@....com>
>Cc: shawnguo@...nel.org; s.hauer@...gutronix.de;
>linux-kernel@...r.kernel.org; linux@...pel-privat.de; dl-linux-imx
><linux-imx@....com>; kernel@...gutronix.de; festevam@...il.com;
>linux-arm-kernel@...ts.infradead.org; linux-i2c@...r.kernel.org
>Subject: Re: [EXT] Re: [PATCH] i2c: imx: Defer probing if EDMA not available
>
>Caution: EXT Email
>
>On Wed, Dec 11, 2019 at 11:22:00AM +0000, Peng Ma wrote:
>>
>>
>> >-----Original Message-----
>> >From: Russell King - ARM Linux admin <linux@...linux.org.uk>
>> >Sent: 2019年12月11日 18:44
>> >To: Peng Ma <peng.ma@....com>
>> >Cc: festevam@...il.com; s.hauer@...gutronix.de;
>> >linux-kernel@...r.kernel.org; linux@...pel-privat.de; dl-linux-imx
>> ><linux-imx@....com>; kernel@...gutronix.de; shawnguo@...nel.org;
>> >linux-arm-kernel@...ts.infradead.org; linux-i2c@...r.kernel.org
>> >Subject: Re: [EXT] Re: [PATCH] i2c: imx: Defer probing if EDMA not
>> >available
>> >
>> >Caution: EXT Email
>> >
>> >On Wed, Dec 11, 2019 at 10:25:26AM +0000, Peng Ma wrote:
>> >> Hi Russell,
>> >>
>> >> I am sorry to reply late, thanks for your patient reminding, Please
>> >> see my comments inline.
>> >>
>> >> Best Regards,
>> >> Peng
>> >> >-----Original Message-----
>> >> >From: Russell King - ARM Linux admin <linux@...linux.org.uk>
>> >> >Sent: 2019年11月28日 18:06
>> >> >To: Peng Ma <peng.ma@....com>
>> >> >Cc: linux@...pel-privat.de; kernel@...gutronix.de;
>> >> >shawnguo@...nel.org; s.hauer@...gutronix.de;
>> >> >linux-kernel@...r.kernel.org; dl-linux-imx <linux-imx@....com>;
>> >> >festevam@...il.com; linux-arm-kernel@...ts.infradead.org;
>> >> >linux-i2c@...r.kernel.org
>> >> >Subject: [EXT] Re: [PATCH] i2c: imx: Defer probing if EDMA not
>> >> >available
>> >> >
>> >> >Caution: EXT Email
>> >> >
>> >> >On Wed, Nov 27, 2019 at 07:12:09AM +0000, Peng Ma wrote:
>> >> >> EDMA may be not available or defered due to dependencies on
>> >> >> other modules, If these scenarios is encountered, we should defer
>probing.
>> >> >
>> >> >This has been tried before in this form, and it causes regressions.
>> >> >
>> >> >> Signed-off-by: Peng Ma <peng.ma@....com>
>> >> >> ---
>> >> >>  drivers/i2c/busses/i2c-imx.c | 16 +++++++++++-----
>> >> >>  1 file changed, 11 insertions(+), 5 deletions(-)
>> >> >>
>> >> >> diff --git a/drivers/i2c/busses/i2c-imx.c
>> >> >> b/drivers/i2c/busses/i2c-imx.c index 40111a3..c2b0693 100644
>> >> >> --- a/drivers/i2c/busses/i2c-imx.c
>> >> >> +++ b/drivers/i2c/busses/i2c-imx.c
>> >> >> @@ -369,8 +369,8 @@ static void i2c_imx_reset_regs(struct
>> >> >> imx_i2c_struct *i2c_imx)  }
>> >> >>
>> >> >>  /* Functions for DMA support */ -static void
>> >> >> i2c_imx_dma_request(struct imx_i2c_struct *i2c_imx,
>> >> >> -                                             dma_addr_t
>> >> >phy_addr)
>> >> >> +static int i2c_imx_dma_request(struct imx_i2c_struct *i2c_imx,
>> >> >> +                            dma_addr_t phy_addr)
>> >> >>  {
>> >> >>       struct imx_i2c_dma *dma;
>> >> >>       struct dma_slave_config dma_sconfig; @@ -379,7 +379,7 @@
>> >> >> static void i2c_imx_dma_request(struct imx_i2c_struct *i2c_imx,
>> >> >>
>> >> >>       dma = devm_kzalloc(dev, sizeof(*dma), GFP_KERNEL);
>> >> >>       if (!dma)
>> >> >> -             return;
>> >> >> +             return -ENOMEM;
>> >> >>
>> >> >>       dma->chan_tx = dma_request_chan(dev, "tx");
>> >> >>       if (IS_ERR(dma->chan_tx)) { @@ -424,7 +424,7 @@ static
>> >> >> void i2c_imx_dma_request(struct
>> >> >imx_i2c_struct *i2c_imx,
>> >> >>       dev_info(dev, "using %s (tx) and %s (rx) for DMA transfers\n",
>> >> >>               dma_chan_name(dma->chan_tx),
>> >> >> dma_chan_name(dma->chan_rx));
>> >> >>
>> >> >> -     return;
>> >> >> +     return 0;
>> >> >>
>> >> >>  fail_rx:
>> >> >>       dma_release_channel(dma->chan_rx);
>> >> >> @@ -432,6 +432,8 @@ static void i2c_imx_dma_request(struct
>> >> >imx_i2c_struct *i2c_imx,
>> >> >>       dma_release_channel(dma->chan_tx);
>> >> >>  fail_al:
>> >> >>       devm_kfree(dev, dma);
>> >> >> +
>> >> >> +     return ret;
>> >> >
>> >> >Some platforms don't have EDMA.  Doesn't this force everyone who
>> >> >wants I2C to have DMA?  The last attempt at this had:
>> >> >
>> >> >        /* return successfully if there is no dma support */
>> >> >        return ret == -ENODEV ? 0 : ret;
>> >> >
>> >> >here because of exactly this.
>> >> >
>> >> >>  }
>> >> >>
>> >> >>  static void i2c_imx_dma_callback(void *arg) @@ -1605,10
>> >> >> +1607,14 @@ static int i2c_imx_probe(struct platform_device *pdev)
>> >> >>       dev_info(&i2c_imx->adapter.dev, "IMX I2C adapter
>> >> >> registered\n");
>> >> >>
>> >> >>       /* Init DMA config if supported */
>> >> >> -     i2c_imx_dma_request(i2c_imx, phy_addr);
>> >> >> +     ret = i2c_imx_dma_request(i2c_imx, phy_addr);
>> >> >> +     if (ret == -EPROBE_DEFER)
>> >> >> +             goto i2c_adapter_remove;
>> >> >
>> >> >This happens _after_ the adapter has been published to the rest of
>> >> >the
>> >kernel.
>> >> >Claiming resources after publication is racy - the adapter may be
>> >> >in use by a request at this point.  Secondly, there's been
>> >> >problems with this causing regressions when EDMA is built as a
>> >> >module and i2c-imx is
>> >built-in.
>> >> >
>> >> >See e8c220fac415 ("Revert "i2c: imx: improve the error handling in
>> >> >i2c_imx_dma_request()"") when exactly what you're proposing was
>> >> >tried and ended up having to be reverted.
>> >> >
>> >> >AFAIK nothing has changed since, so merely reinstating the known
>> >> >to be broken code, thereby reintroducing the same (and more)
>> >> >problems, isn't going to be acceptable.
>> >> >
>> >> >Sorry, but this gets a big NAK from me.
>> >> >
>> >> [Peng Ma] I saw the revert commit e8c220fac415 and understand your
>> >concerns.
>> >> I scan the i2c-imx.c driver, All platforms that use i2c driver and
>> >> support dma use an eDMA engine, So I change the code(compare with
>> >> last
>> >patch) as follows, please review and give me your precious comments.
>> >> Thanks very much.
>> >>
>> >> diff --git a/drivers/i2c/busses/i2c-imx.c
>> >> b/drivers/i2c/busses/i2c-imx.c index 12f7934fddb4..6cafee52dd67
>> >> 100644
>> >> --- a/drivers/i2c/busses/i2c-imx.c
>> >> +++ b/drivers/i2c/busses/i2c-imx.c
>> >> @@ -1605,8 +1605,11 @@ static int i2c_imx_probe(struct
>> >> platform_device
>> >> *pdev)
>> >>
>> >>         /* Init DMA config if supported */
>> >>         ret = i2c_imx_dma_request(i2c_imx, phy_addr);
>> >> -       if (ret == -EPROBE_DEFER)
>> >> +       if (ret == -EPROBE_DEFER) {
>> >> +#if    IS_BUILTIN(CONFIG_FSL_EDMA)
>> >>                 goto i2c_adapter_remove;
>> >> +#endif
>> >> +       }
>> >
>> >You haven't understood _why_ the problem occurs, you're just
>> >attempting to patch around it. You're hacking the code, rather than
>engineering the code.
>> >
>> >The infinite deferred probe occurs because:
>> >
>> >- i2c-imx is attempted to be probed.
>> >- i2c-imx sets up the hardware, and then calls
>> >  i2c_add_numbered_adapter()
>> >- i2c_add_numbered_adapter() publishes the bus to the world, and then
>> >  searches DT for any children to create - and it finds some and
>> >  creates them.
>> >- the children devices are matched to their drivers, which bind.
>> >This
>> >  triggers a deferred probe to be scheduled.
>> >- back in the i2c-imx driver, we get to i2c_imx_dma_request(), which
>> >  fails, and you return -EPROBE_DEFER.
>> >- the i2c-imx driver probe actions are unwound, and probe exits.
>> >- the driver core processes the deferred probe request, finds the
>> >  i2c-imx device(s) on the deferred probe list, and attempts to
>> >  probe them.  Goto the top of this list.
>> >
>> [Peng Ma] Thanks for your quick reply, No, I don't think so, when
>first,second,third...... time probe failed, the i2c_del_adapter will be called(it
>will remove the i2c children device). I think if We build-in EDMA, after EDMA
>probe successful, the deffer probe of i2c will probe with no return
>-EPROBE_DEFER.
>
>Yes, i2c_del_adapter will be called, but that is neither here nor there.
>The deferred probe is triggered by _any_ driver binding.  The facts are:
>
>i2c_add_numbered_adapter() creates devices.
>These new devices get bound to drivers.
>As soon as any one of those devices binds to a driver, deferred probing is
>triggered.
>When i2c_imx_probe() returns -EPROBE_DEFER, it will be added to the list of
>devices to be re-probed by the deferred probing.
>
>> So you say " Goto the top of this list " just i2c drive probe failed
>> with i2c_imx_dma_request() return -EPROBE_DEFER, If the EDMA build-in
>and probe successful this case not happened. Now I am worried about EDMA
>failed to probe, your case is correct.
>
>You are assuming that EDMA has successfully probed. What if EDMA hasn't
>been probed yet, because it has been deferred for some other reason (e.g.
>a clock)?
>
>The fact is, the way i2c-imx is structured at present, it is unsafe to propagate
>the EPROBE_DEFER error code from i2c_imx_dma_request() under ANY
>CIRCUMSTANCES.
>
>> >If, for whatever reason, i2c_imx_dma_request() ever returns
>> >-EPROBE_DEFER, the above loop WILL happen.
>> >
>> >The FUNDAMENTAL rule of kernel programming is that you do NOT publish
>> >before you have completed setup.  i2c-imx violates that rule as the
>> >probe function is ordered at present.
>> >
>> [Peng Ma] Yes, I agree, but kernel provide the deffer probe and for the
>platform devices we don't decide who probe first.
>
>So, because the kernel provides a facility, you think it's fine to create infinite
>loops using it?
>
>> >i2c-imx has been written for i2c_imx_dma_request() to be safe to call
>> >after the device has been published, but with the current probe
>> >function order, it is unsafe to propagate the EPROBE_DEFER return value for
>the reason above.
>> >For the reason the original attempt got reverted.
>> >
>> >So, if you want to do this (and yes, I'd also encourage it to be
>> >conditional on EDMA being built-in, as I2C is commonly used as a way
>> >to get at RTCs, which are read before kernel modules can be loaded)
>> >then you MUST move
>> >i2c_imx_dma_request() before
>> >i2c_add_numbered_adapter() to avoid the infinite loop.
>> >
>> [Peng Ma] To do this, the i2c devices not probe and i2c adapter not register
>before edma probe.
>
>Which is the correct behaviour, rather than having the kernel cycle through
>creating i2c devices, probing i2c drivers, tearing down the i2c devices and
>repeating endlessly.
>
>Until you see this, sorry, no, you can't propagate the return value from
>i2c_imx_dma_request().  We've tried it, it's caused regressions, and a
>problem has been identified that you don't seem to be willing to recognise _as_
>a serious problem with the approach you're trying to re-implement.
>
>--
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