netdev - Re: [PATCH net v2 1/2] ptp: ocp: adjust sysfs entries to expose tty information

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Message-ID: <831c8bb5-fb39-439b-9ffa-3f55620cb6b3@kernel.org>
Date: Wed, 14 Aug 2024 12:30:04 +0200
From: Jiri Slaby <jirislaby@...nel.org>
To: Vadim Fedorenko <vadim.fedorenko@...ux.dev>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 Vadim Fedorenko <vadfed@...a.com>
Cc: Jakub Kicinski <kuba@...nel.org>,
 Jonathan Lemon <jonathan.lemon@...il.com>, netdev@...r.kernel.org
Subject: Re: [PATCH net v2 1/2] ptp: ocp: adjust sysfs entries to expose tty
 information

On 14. 08. 24, 10:37, Vadim Fedorenko wrote:
> On 14/08/2024 06:00, Jiri Slaby wrote:
>> On 13. 08. 24, 20:24, Vadim Fedorenko wrote:
>>> On 13/08/2024 10:33, Greg Kroah-Hartman wrote:
>>>> On Mon, Aug 05, 2024 at 03:04:59PM -0700, Vadim Fedorenko wrote:
>>>>> Starting v6.8 the serial port subsystem changed the hierarchy of 
>>>>> devices
>>>>> and symlinks are not working anymore. Previous discussion made it 
>>>>> clear
>>>>> that the idea of symlinks for tty devices was wrong by design. 
>>>>> Implement
>>>>> additional attributes to expose the information. Fixes tag points 
>>>>> to the
>>>>> commit which introduced the change.
>>>>>
>>>>> Fixes: b286f4e87e32 ("serial: core: Move tty and serdev to be 
>>>>> children of serial core port device")
>>>>> Signed-off-by: Vadim Fedorenko <vadfed@...a.com>
>>>>> ---
>>>>>   drivers/ptp/ptp_ocp.c | 68 
>>>>> +++++++++++++++++++++++++++++++++----------
>>>>>   1 file changed, 52 insertions(+), 16 deletions(-)
>>>>>
>>>>> diff --git a/drivers/ptp/ptp_ocp.c b/drivers/ptp/ptp_ocp.c
>>>>> index ee2ced88ab34..7a5026656452 100644
>>>>> --- a/drivers/ptp/ptp_ocp.c
>>>>> +++ b/drivers/ptp/ptp_ocp.c
>>>>> @@ -3346,6 +3346,55 @@ static EXT_ATTR_RO(freq, frequency, 1);
>>>>>   static EXT_ATTR_RO(freq, frequency, 2);
>>>>>   static EXT_ATTR_RO(freq, frequency, 3);
>>>>> +static ssize_t
>>>>> +ptp_ocp_tty_show(struct device *dev, struct device_attribute 
>>>>> *attr, char *buf)
>>>>> +{
>>>>> +    struct dev_ext_attribute *ea = to_ext_attr(attr);
>>>>> +    struct ptp_ocp *bp = dev_get_drvdata(dev);
>>>>> +    struct ptp_ocp_serial_port *port;
>>>>> +
>>>>> +    port = (void *)((uintptr_t)bp + (uintptr_t)ea->var);
>>>>
>>>> That's insane pointer math, how do we know this is correct?
>>>>
>>>>> +    return sysfs_emit(buf, "ttyS%d", port->line);
>>>>> +}
>>>>> +
>>>>> +static umode_t
>>>>> +ptp_ocp_timecard_tty_is_visible(struct kobject *kobj, struct 
>>>>> attribute *attr, int n)
>>>>> +{
>>>>> +    struct ptp_ocp *bp = dev_get_drvdata(kobj_to_dev(kobj));
>>>>> +    struct ptp_ocp_serial_port *port;
>>>>> +    struct device_attribute *dattr;
>>>>> +    struct dev_ext_attribute *ea;
>>>>> +
>>>>> +    if (strncmp(attr->name, "tty", 3))
>>>>> +        return attr->mode;
>>>>> +
>>>>> +    dattr = container_of(attr, struct device_attribute, attr);
>>>>> +    ea = container_of(dattr, struct dev_ext_attribute, attr);
>>>>> +    port = (void *)((uintptr_t)bp + (uintptr_t)ea->var);
>>>>
>>>> That's crazy pointer math, how are you ensured that it is correct?  Why
>>>> isn't there a container_of() thing here instead?
>>>
>>> Well, container_of cannot be used here because the attributes are static
>>> while the function reads dynamic instance. The only values that are
>>> populated into the attributes of the group are offsets.
>>> But I can convert it to a helper which will check that the offset 
>>> provided is the real offset of the structure we expect. And it could 
>>> be reused in both "is_visible" and "show" functions.
>>
>> Strong NACK against this approach.
>>
>> What about converting those 4 ports into an array and adding an enum { 
>> PORT_GNSS, POTR_GNSS2, PORT_MAC, PORT_NMEA }?
> 
> Why is it a problem? I don't see big difference between these 2
> implementations:
> 
> struct ptp_ocp_serial_port *get_port(struct ptp_ocp *bp, void *offset)
> {
>      switch((uintptr_t)offset) {
>          case offsetof(struct ptp_ocp, gnss_port):
>              return &bp->gnss_port;
>          case offsetof(struct ptp_ocp, gnss2_port):
>              return &bp->gnss2_port;
>          case offsetof(struct ptp_ocp, mac_port):
>              return &bp->mac_port;
>          case offsetof(struct ptp_ocp, nmea_port):
>              return &bp->nmea_port;
>      }
>      return NULL;
> }
> 
> and:
> 
> struct ptp_ocp_serial_port *get_port(struct ptp_ocp *bp, void *offset)
> {
>      switch((enum port_type)offset) {
>          case PORT_GNSS:
>              return &bp->tty_port[PORT_GNSS];
>          case PORT_GNSS2:
>              return &bp->tty_port[PORT_GNSS2];
>          case PORT_MAC:
>              return &bp->tty_port[PORT_MAC];
>          case PORT_NMEA:
>              return &bp->tty_port[PORT_NMEA];
>      }
>      return NULL;
> }
> 
> The second option will require more LoC to change the initialization
> part of the driver, but will not simplify the access.
> If you suggest to use enum value directly, without the check, then
> it will not solve the problem of checking the boundary, which Greg
> refers to AFAIU.

Why do you need this get_port() here at all? Simply doing 
bp->tty_port[ea->var] as in already present attrs will do the job, right?

thanks,
-- 
js
suse labs