Date:   Mon, 24 Jan 2022 13:35:35 +0100
From:   Jan Dąbroś <jsd@...ihalf.com>
To:     Hans de Goede <hdegoede@...hat.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-i2c <linux-i2c@...r.kernel.org>,
        Jarkko Nikula <jarkko.nikula@...ux.intel.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Wolfram Sang <wsa@...nel.org>,
        Raul E Rangel <rrangel@...omium.org>,
        Marcin Wojtas <mw@...ihalf.com>,
        Grzegorz Jaszczyk <jaz@...ihalf.com>, upstream@...ihalf.com,
        mario.limonciello@....com
Subject: Re: [PATCH 2/2] i2c: designware: Add AMD PSP I2C bus support

pt., 21 sty 2022 o 11:32 Hans de Goede <hdegoede@...hat.com> napisał(a):
>
> Hi Jan,
>
> On 1/21/22 10:59, Jan Dąbroś wrote:
> > (...)
> >
> >>> --- /dev/null
> >>> +++ b/drivers/i2c/busses/i2c-designware-amdpsp.c
> >>> @@ -0,0 +1,357 @@
> >>> +// SPDX-License-Identifier: GPL-2.0
> >>> +
> >>> +#include <asm/msr.h>
> >>> +#include <linux/i2c.h>
> >>> +#include <linux/psp-sev.h>
> >>> +
> >>> +#include "i2c-designware-core.h"
> >>
> >> So all the stuff starting here:
> >>
> >>> +
> >>> +#define MSR_AMD_PSP_ADDR     0xc00110a2
> >>> +#define PSP_MBOX_OFFSET              0x10570
> >>> +#define PSP_CMD_TIMEOUT_MS   500
> >>> +
> >>> +#define PSP_I2C_REQ_BUS_CMD          0x64
> >>> +#define PSP_I2C_REQ_RETRY_CNT                10
> >>> +#define PSP_I2C_REQ_RETRY_DELAY_MSEC 50
> >>> +#define PSP_I2C_REQ_STS_OK           0x0
> >>> +#define PSP_I2C_REQ_STS_BUS_BUSY     0x1
> >>> +#define PSP_I2C_REQ_STS_INV_PARAM    0x3
> >>> +
> >>> +union psp_req_buffer_hdr {
> >>> +     struct {
> >>> +             u32 total_size;
> >>> +             u32 status;
> >>> +     } __packed;
> >>> +     u64 hdr_val;
> >>> +};
> >>> +
> >>> +enum psp_i2c_req_type {
> >>> +     PSP_I2C_REQ_ACQUIRE,
> >>> +     PSP_I2C_REQ_RELEASE,
> >>> +     PSP_I2C_REQ_MAX,
> >>> +};
> >>> +
> >>> +struct psp_i2c_req {
> >>> +     union psp_req_buffer_hdr hdr;
> >>> +     enum psp_i2c_req_type type;
> >>> +} __packed __aligned(32);
> >>> +
> >>> +union psp_mbox_cmd_reg {
> >>> +     struct psp_mbox_cmd_fields {
> >>> +             u16 mbox_status;
> >>> +             u8 mbox_cmd;
> >>> +             u8 reserved:6;
> >>> +             u8 recovery:1;
> >>> +             u8 ready:1;
> >>> +     } __packed fields;
> >>> +     u32 val;
> >>> +};
> >>> +
> >>> +struct psp_mbox {
> >>> +     union psp_mbox_cmd_reg fields;
> >>> +     uintptr_t i2c_req_addr;
> >>> +} __packed;
> >>> +
> >>> +static DEFINE_MUTEX(psp_i2c_access_mutex);
> >>> +static unsigned long psp_i2c_sem_acquired;
> >>> +static void __iomem *mbox_iomem;
> >>> +static u32 psp_i2c_access_count;
> >>> +static bool psp_i2c_mbox_fail;
> >>> +static struct device *psp_i2c_dev;
> >>> +
> >>> +static int psp_get_mbox_addr(unsigned long *mbox_addr)
> >>> +{
> >>> +     unsigned long long psp_mmio;
> >>> +
> >>> +     if (rdmsrl_safe(MSR_AMD_PSP_ADDR, &psp_mmio))
> >>> +             return -EIO;
> >>> +
> >>> +     *mbox_addr = (unsigned long)(psp_mmio + PSP_MBOX_OFFSET);
> >>> +
> >>> +     return 0;
> >>> +}
> >>> +
> >>> +static int psp_mbox_probe(void)
> >>> +{
> >>> +     unsigned long mbox_addr;
> >>> +
> >>> +     if (psp_get_mbox_addr(&mbox_addr))
> >>> +             return -1;
> >>> +
> >>> +     mbox_iomem = ioremap(mbox_addr, sizeof(struct psp_mbox));
> >>> +     if (!mbox_iomem)
> >>> +             return -ENOMEM;
> >>> +
> >>> +     return 0;
> >>> +}
> >>> +
> >>> +/* Recovery field should be equal 0 to start sending commands */
> >>> +static int psp_check_mbox_recovery(struct psp_mbox *mbox)
> >>> +{
> >>> +     union psp_mbox_cmd_reg tmp = {0};
> >>> +
> >>> +     tmp.val = readl(&mbox->fields.val);
> >>> +     return !!tmp.fields.recovery;
> >>> +}
> >>> +
> >>> +static int psp_wait_cmd(struct psp_mbox *mbox)
> >>> +{
> >>> +     union psp_mbox_cmd_reg expected = { .val = 0 };
> >>> +     u32 tmp;
> >>> +
> >>> +     /* Expect mbox_cmd to be cleared and ready bit to be set by PSP */
> >>> +     expected.fields.ready = 1;
> >>> +
> >>> +     return readl_poll_timeout(&mbox->fields.val, tmp, (tmp == expected.val),
> >>> +                               0, 1000 * PSP_CMD_TIMEOUT_MS);
> >>> +}
> >>> +
> >>> +/* Status equal to 0 means that PSP succeed processing command */
> >>> +static int psp_check_mbox_sts(struct psp_mbox *mbox)
> >>> +{
> >>> +     union psp_mbox_cmd_reg cmd_reg = {0};
> >>> +
> >>> +     cmd_reg.val = readl(&mbox->fields.val);
> >>> +     return cmd_reg.fields.mbox_status;
> >>> +}
> >>> +
> >>> +static int psp_send_cmd(struct psp_i2c_req *req)
> >>> +{
> >>> +     struct psp_mbox *mbox = (struct psp_mbox *)mbox_iomem;
> >>> +     union psp_mbox_cmd_reg cmd_reg = {0};
> >>> +
> >>> +     if (psp_check_mbox_recovery(mbox))
> >>> +             return -EIO;
> >>> +
> >>> +     if (psp_wait_cmd(mbox))
> >>> +             return -EBUSY;
> >>> +
> >>> +     /* Fill address of command-response buffer */
> >>> +     writeq((uintptr_t)__psp_pa((void *)req), &mbox->i2c_req_addr);
> >>> +
> >>> +     /* Write command register to trigger processing */
> >>> +     cmd_reg.fields.mbox_cmd = PSP_I2C_REQ_BUS_CMD;
> >>> +     writel(cmd_reg.val, &mbox->fields.val);
> >>> +
> >>> +     if (psp_wait_cmd(mbox))
> >>> +             return -ETIMEDOUT;
> >>> +
> >>> +     if (psp_check_mbox_sts(mbox))
> >>> +             return -EIO;
> >>> +
> >>> +     return 0;
> >>> +}
> >>
> >> Through here seems to all be generic code for accessing
> >> the AMD PSP. To me this seems like something which belongs
> >> in a separate AMD-PSP-mbox driver/lib, which can then be
> >> shared between other kernel drivers which may also want
> >> to access PSP.
> >
> > I see your point clearly and actually it is not an accident that I've
> > put all PSP-mailbox methods in one "block". They are logically
> > different than the rest of i2c-adapter specific methods.
> >
> > That being said, above PSP mailbox was created by AMD solely for the
> > purpose of i2c_arbitration. It has its own set of commands and
> > specific format of the command-response buffer. Thus it is not and it
> > won't be generic in the future. There are already upstreamed drivers
> > from AMD (under drivers/crypto/ccp/) which made use of PSP, however
> > their channel of communication looks completely different than the
> > very simple i2c_arbitration model implemented above.
> >
> > Because of this I'm treating this as an i2c_semaphore-related code and
> > putting this in this module. In my opinion moving this into some
> > separate driver (which will be actually used only here) makes code
> > less clear. But let's also hear some voice from AMD.
>
> Since as you say this mailbox is special and only for i2c-arbitration,
> keeping it inside this patch / .c file is fine.
>
> >
> >>
> >> Sorta like the generic iosf_mbi_read() and
> >> iosf_mbi_write() functions from:
> >>
> >> arch/x86/platform/intel/iosf_mbi.c
> >>
> >> used on the Intel chips, which are also used outside of
> >> the I2C bus-locking code.
> >>
> >> This is also one of the reasons why I think it would be
> >> good to get some AMD folks involved in this, since they
> >> may be aware of other drivers which also need to access
> >> the PSP mbox.
> >>
> >
> > Right, I'm adding mario.limonciello@....com to the CC, so that he can comment.
> >
> > (...)
> >
> >>> +/*
> >>> + * Locking methods are based on the default implementation from
> >>> + * drivers/i2c/i2c-core.base.c, but with psp acquire and release operations
> >>> + * added. With this in place we can ensure that i2c clients on the bus shared
> >>> + * with psp are able to lock HW access to the bus for arbitrary number of
> >>> + * operations - that is e.g. write-wait-read.
> >>> + */
> >>> +static void i2c_adapter_dw_psp_lock_bus(struct i2c_adapter *adapter,
> >>> +                                     unsigned int flags)
> >>> +{
> >>> +     psp_acquire_i2c_bus();
> >>> +     rt_mutex_lock_nested(&adapter->bus_lock, i2c_adapter_depth(adapter));
> >>
> >> This does not do what you think it does and you will still deadlock
> >> when things nest because of someone taking the bus-lock and then
> >> the main i2c-designware transfer function calling the acquire_lock
> >> callback.
> >
> > I haven't used rt_mutex_lock_nested() with the intent to prevent me
> > from deadlock when i2c-designware calls acquire_lock with bus-lock
> > already taken. This is a method copied from
> > drivers/i2c/i2c-core-base.c (BTW, I have a typo in above comment).
> > This is the default implementation applied by i2c-core when particular
> > adapter doesn't register its own locking callbacks - thus it is called
> > for i2c-designware for all platforms.
> >
> > In case of this driver internal i2c-designware acquire_lock() is equal
> > to psp_acquire_i2c_bus(). In other words, bus-level lock
> > i2c_adapter_dw_psp_lock_bus() is a superset of internal adapter's
> > acquire_lock().
>
> Ah I missed that this is just mimicking the core functions +
> an extra call to psp_acquire_i2c_bus().
>
> I assumed that the dwc->acquire callback path was also taking
> the mutex and I thought you had fallen for the _nested meaning
> something different then it does, my bad.
>
> > In order to prevent deadlock which you are talking about, I'm using
> > reference lock counter inside psp_acquire_i2c_bus() thus it is safe to
> > invoke acquire_lock() when bus-lock is already taken.
>
> Ah good, that is pretty much is the same as what the Bay Trail code
> is doing.
>
> >
> >>
> >> The _nested postfix is only for the lockdep lock-debugger, this
> >> actually turns into a regular mutex_lock when lockdep is not enabled:
> >>
> >> #ifdef CONFIG_DEBUG_LOCK_ALLOC
> >> extern void rt_mutex_lock_nested(struct rt_mutex *lock, unsigned int subclass);
> >> #define rt_mutex_lock(lock) rt_mutex_lock_nested(lock, 0)
> >> #else
> >> extern void rt_mutex_lock(struct rt_mutex *lock);
> >> #define rt_mutex_lock_nested(lock, subclass) rt_mutex_lock(lock)
> >> #endif
> >>
> >> The _nested postfix as such is only to tell the lockdep code that
> >> even though it seems we are trying to take the same mutex twice
> >> since in both cases it is of i2c_adapter.rt_mutex "lock class"
> >> that we are sure it is never the same i2c_adapter (but rather
> >> one which always gets called in a nested fashion from another
> >> i2c_adapter).
> >>
> >> IOW this only disables a false-positive lockdep warning, it does
> >> not allow taking the same mutex twice, you will still hang on
> >> the second mutex_lock call on the same lock.
> >
> > Thanks for the technical background about rt_mutex_lock_nested. I
> > think we should keep using it as is, since as I wrote above I don't
> > have any reasoning to modify it here.
>
> Ack, now that my misreading of the code has been cleared up
> I agree.
>
> >> Also I don't think you are allowed to use the bus_locking code
> >> like this. The i2c bus-locking code is intended to deal with
> >> busses which have muxes in them, where the mux must be set
> >> to the right branch of the bus to reach the client and then
> >> not be changed during the transfer to that client.
> >>
> >> So i2c-client drivers are never supposed to directly call
> >> the bus-locking functions.
> >
> > I think you are not correct here. There are examples of i2c-clients
> > which are using i2c bus_locking for the purpose of locking adapter for
> > the bunch of i2c transactions.
> >
> > As an example let's take drivers/char/tpm/tpm_tis_i2c_cr50.c. It
> > operates in write-wait-read model and there is i2c_lock_bus() call
> > used to ensure that bus won't be released -
> > https://github.com/torvalds/linux/blob/master/drivers/char/tpm/tpm_tis_i2c_cr50.c#L202.
> >
> > Similar model is followed in drivers/char/tpm/tpm_i2c_infineon.c and
> > couple of other i2c-client drivers.
>
> Ah I see, interesting (live and learn).
>
> But this is then combined with using the special __i2c_transfer()
> function for the actual i2c reads/writes, since using the regular
> i2c_transfer() function after already taking the lock would deadlock.

Correct. In other words, if i2c-client wants to block the bus/adapter
for more than one transaction it must use some special methods. This
isn't changed with my patchset. If one is using "normal"
i2c_transfer(), we should be on the safe side, nothing will change
from the i2c-client point of view. The same if one is using
__i2c_transfer().

>
> There is a similar unlocked raw __i2c_smbus_xfer(), but as the
> comment in include/linux/i2c.h above the locked i2c_smbus_xfer() says:
>
> /* This is the very generalized SMBus access routine. You probably do not
>    want to use this, though; one of the functions below may be much easier,
>    and probably just as fast.
>    Note that we use i2c_adapter here, because you do not need a specific
>    smbus adapter to call this function. */
> s32 i2c_smbus_xfer(...);
>
> So in this case a driver cannot use the usual
> i2c_smbus_read_byte/word/byte_data/word_data() helpers and
> the same for writes. Also using an i2c_regmap (which is used
> in a ton of places like PMIC drivers) will not work this way.

Right, however this behavior is not altered by my patch. I just wanted
to ensure that drivers which are already using i2c bus_locking will
still work as expected.

> So yes you can use i2c_bus_lock() for this; but only if all the
> drivers where you want to do that limit themselves to
> __i2c_transfer() and __i2c_smbus_xfer() calls and/or are
> rewritten to only use those.

My goal is to not modify current behavior, that is - we don't need to
modify clients' drivers and extra quirks applied by amdpsp semaphore
will be "transparent" for them. IOW, switch from generic
i2c-designware to i2c-designware-amdpsp should be invisible from the
i2c bus perspective for i2c-clients.

Best Regards,
Jan

> >> This is why in the Bay Trail case we have i2c-drivers
> >> directly calling iosf_mbi_block_punit_i2c_access() and
> >> iosf_mbi_unblock_punit_i2c_access() to lock the bus
> >> for multiple i2c-transfers. We can get away with this there
> >> because the bus in question is only used to access the
> >> PMIC and that PMIC is only used on Bay Trail (and CHT)
> >> boards, so the PMIC drivers can just hard-code these
> >> calls.
> >>
> >> If you need to take the PSP I2C semaphore for multiple
> >> transfers in some generic drivers, then I guess that the
> >> i2c-subsys will need to get some new i2c_adapter callbacks
> >> to acquire / release the bus for i2c-controllers where
> >> the bus/controller is shared with some co-processor like
> >> in the PSP case.
> >
> > This is exactly my intention to support generic i2c-clients drivers
> > without them being aware that i2c-adapter above is using some
> > semaphore/arbitration. Hopefully you can agree with me that currently
> > available bus_locking can be used and is enough for this purpose.
>
> It can be used, but with limitations, see above.
>
> >
> >> Also note that iosf_mbi_block_punit_i2c_access() and
> >> iosf_mbi_unblock_punit_i2c_access() do their own
> >> ref/lock-counting to allow calling them multiple times and
> >> the first block call takes the bus and the last unblock
> >> call releases it.
> >
> > This is exactly what I was talking about above and also implemented
> > within psp_acquire_i2c_bus() and psp_release_i2c_bus().
>
> Right, I was to quick in skimming over your code when
> I wrote down my concerns about there being a deadlock
> there, sorry.
>
> Regards,
>
> Hans
>

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