Message-ID: <2209854.PYKUYFuaPT@fw-rgant>
Date: Thu, 20 Feb 2025 17:40:13 +0100
From: Romain Gantois <romain.gantois@...tlin.com>
To: Cosmin Tanislav <demonsingur@...il.com>
Cc: Tomi Valkeinen <tomi.valkeinen@...asonboard.com>,
 Luca Ceresoli <luca.ceresoli@...tlin.com>,
 Wolfram Sang <wsa+renesas@...g-engineering.com>, linux-i2c@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] i2c: atr: add passthrough flag

On mercredi 19 février 2025 11:22:12 heure normale d’Europe centrale Cosmin 
Tanislav wrote:
> On 2/19/25 11:52 AM, Romain Gantois wrote:
> > Hello Cosmin,
> > 
> > On lundi 3 février 2025 13:15:17 heure normale d’Europe centrale Cosmin
> > 
> > Tanislav wrote:
> >> Some I2C ATRs can have other I2C ATRs as children. The I2C messages of
> >> the child ATRs need to be forwarded as-is since the parent I2C ATR can
> >> only do address remapping for the direct children.
> >> 
> >> In the case of GMSL, the deserializer I2C ATR actually doesn't have I2C
> >> address remapping hardware capabilities, but it is able to select which
> >> GMSL link to talk to, allowing it to change the address of the
> >> serializer.
> >> 
> >> The child ATRs need to have their alias pools defined in such a way to
> >> prevent overlapping addresses between them, but there's no way around
> >> this without orchestration between multiple ATR instances.
> >> 
> >> To allow for this use-case, add a flag that allows unmapped addresses
> >> to be passed through, since they are already remapped by the child ATRs,
> >> and disables dynamic remapping, since devices that need passthrough
> >> messages to be forwarded as-is, can only handle remapping for their
> >> direct children.
> >> 
> >> There's no case where a non-remapped address will hit the parent ATR.
> > 
> > I'm having trouble understanding this, because it seems like there's a
> > 
> > contradiction with your previous statement:
> >> add a flag that allows unmapped addresses to be passed through
> > 
> > Unmapped addresses are "non-remapped" by definition right? And they can
> > hit the parent ATR since we're adding a flag to allow them to pass
> > through...
> Non-remapped address means addresses that have never been remapped, on
> any ATR instance, not on the parent one.
> 

Ah I see, that makes more sense.

> It's impossible for non-remapped addresses to reach the parent ATR
> since the direct children are remapped by the parent ATR and the
> children of the child ATRs are remapped by the child ATRs.
> 
> Unampped address means addresses that are not remapped on the current
> ATR instance (the parent one, in this case, since that's where the flag
> is supposed to be used).
> 
> I agree that my explanation was a bit confusing.
> 

Well ATR's aren't the most straightforward kind of component anyway, but in 
any case I definitely think that these explanations of the "unmapped" and "non-
remapped" terminology should be in the commit log.
 
> >> Signed-off-by: Cosmin Tanislav <demonsingur@...il.com>
> >> ---
> >> 
> >>   drivers/i2c/i2c-atr.c   | 26 ++++++++++++++++++--------
> >>   include/linux/i2c-atr.h | 20 +++++++++++++++++---
> >>   2 files changed, 35 insertions(+), 11 deletions(-)
> >> 
> >> diff --git a/drivers/i2c/i2c-atr.c b/drivers/i2c/i2c-atr.c
> >> index 13f7e07fd8e87..5f0e8f1cf69f7 100644
> >> --- a/drivers/i2c/i2c-atr.c
> >> +++ b/drivers/i2c/i2c-atr.c
> >> @@ -106,6 +106,7 @@ struct i2c_atr_chan {
> >> 
> >>    * @lock:      Lock for the I2C bus segment (see &struct
> >> 
> >> i2c_lock_operations) * @lock_key:  Lock key for @lock
> >> 
> >>    * @max_adapters: Maximum number of adapters this I2C ATR can have
> >> 
> >> + * @flags:     Flags for ATR
> >> 
> >>    * @alias_pool: Optional common pool of available client aliases
> >>    * @i2c_nb:    Notifier for remote client add & del events
> >>    * @adapter:   Array of adapters
> >> 
> >> @@ -122,6 +123,7 @@ struct i2c_atr {
> >> 
> >>   	struct mutex lock;
> >>   	struct lock_class_key lock_key;
> >>   	int max_adapters;
> >> 
> >> +	u32 flags;
> >> 
> >>   	struct i2c_atr_alias_pool *alias_pool;
> >> 
> >> @@ -241,7 +243,7 @@ static void i2c_atr_release_alias(struct
> >> i2c_atr_alias_pool *alias_pool, u16 ali
> >> 
> >>   /* Must be called with alias_pairs_lock held */
> >>   static struct i2c_atr_alias_pair *
> >> 
> >> -i2c_atr_find_mapping_by_addr(struct i2c_atr_chan *chan, u16 addr)
> >> +i2c_atr_find_mapping_by_addr(struct i2c_atr_chan *chan, u16 addr, bool
> >> new_addr) {
> > 
> > IMO the "new_addr" naming is quite confusing.
> 
> Could you suggest a better name? I picked new_addr since when that flag
> is true, we are mapping a new address, the call is coming from
> i2c_atr_attach_addr().

My issue with this terminology is that "new address" could mean a number of 
things, including "an address that the ATR hasn't encountered before". IMO the 
effects of the flag on the function body should be understandable without having 
to go through all of the function's call sites.

Rather than renaming the flag however, I'd rather we not handle this logic in 
find_mapping_by_addr() at all. If we just search for mappings in 
find_mapping_by_addr() and separate out mapping creation into another function, 
we can avoid the need for such a flag altogether.

> When we're mapping a new address, and we are in passthrough mode, we're
> free to reserve a new alias.
> I've since then modified the conditions slightly to cover an edge case.
> 
> if (!new_addr && (atr->flags & I2C_ATR_PASSTHROUGH))
> 	return NULL;
> 
> ret = i2c_atr_reserve_alias(chan->alias_pool);
> if (ret < 0) {
> 	if (atr->flags & I2C_ATR_PASSTHROUGH)
> 		return NULL;
> 
> 	...
> }
> 
> With this change, if we've made it past the section where we look for
> an existing mapping, and we're not adding a new address, and we're in
> passthrough mode, we return NULL. Passthrough mode shouldn't allow
> aliases to be reserved dynamically, since the hardware can only map
> direct children.
> 
> Also, if we are adding a new address, and we failed to reserve a free
> alias, and we are in passthrough mode, we don't allow replacing an
> existing mapping.
> 
> > After this patch is applied, the expected behavior is:
> > 
> > i2c_atr_find_mapping_by_addr() called from i2c_atr_attach_addr():
> >    1. find existing mapping, return it
> >    2. OR find free alias, create mapping and return it
> >    3. OR remap used alias, return mapping
> >    4. OR fail
> > 
> > i2c_atr_find_mapping_by_addr(), called from anywhere else:
> >     1. find existing mapping, return it
> >     2. OR find free alias, create mapping and return it
> >     3. OR if the ATR has PASSTHROUGH set, fail
> >     4. OR remap used alias, return mapping
> >     5. OR fail
> > 
> > To me, the proposed code doesn't make it immediately obvious why the
> > PASSTHROUGH flag should have anything to do with not attempting alias
> > remapping.
> > 
> > Moreover, if we truly want to ignore *all* unmapped addresses, then
> > shouldn't we also give up on step 2.? (the one that tries to map a free
> > alias to the requested address).
> > 
> > In that case, I think something like this would be clearer:
> > 
> > in  i2c_atr_smbus_xfer() and i2c_atr_map_msgs():
> > 
> > ```
> > #never attempts to create a new mapping, only to find an existing one
> > c2a = i2c_atr_find_mapping_by_addr(chan, msgs[i].addr);
> > if (!c2a) {
> > 
> > 	if (PASSTHROUGH)
> > 	
> > 		# Since passthrough is set, we ignore unmapped addresses
> > 		goto success or whatever;
> > 	
> > 	c2a = i2c_atr_create_mapping(chan, msgs[i].addr);
> > 	if (!c2a)
> > 	
> > 		fail;
> > 
> > }
> > ```
> > 
> > in i2c_atr_attach_addr():
> > 
> > ```
> > c2a = i2c_atr_find_mapping_by_addr(chan, msgs[i].addr);
> > if (!c2a) {
> > 
> > 	c2a = i2c_atr_create_mapping(chan, msgs[i].addr);
> > 	if (!c2a)
> > 	
> > 		fail;
> > 
> > }
> > ```
> > 
> > So what I'm suggesting is to remove all c2a mapping creation logic from
> > find_mapping_by_addr() entirely, and to move it to a separate function.
> > 
> > Please let me know what you think.
> 
> In the case of passthrough ATR, mapping creation should only be allowed
> when direct devices are attached, ie: in the call to
> i2c_atr_find_mapping_by_addr() from i2c_atr_attach_addr().
> If i2c_atr_find_mapping_by_addr() cannot find a free alias in that case,
> it should fail.
> 

I think it would be better if i2c_atr_find_mapping_by_addr() never tried to 
create a new mapping, not even if an alias is available. This would eliminate 
the need for a special flag passed to the function and let 
i2c_atr_attach_addr() handle it's own logic instead.

> Other calls to i2c_atr_find_mapping_by_addr() should either return an
> existing alias or NULL, and not attempt to create a new one or to
> replace an existing one.
> 
> Let me know if my explanations made it clearer and what the you think
> about going forward with this patch. In the meantime, I'll try to see
> how the code looks with splitting creating from finding.
> 

The patch itself looks like a valid idea to me, but indeed I'd really prefer 
to see a solution where mapping creation and finding are completely separated, 
including for cases where a free alias is used to create a mapping.

Thanks,

-- 
Romain Gantois, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com

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