Re: [PATCH RESEND v2 2/7] mfd: cros_ec: Add char dev and virtual dev pointers

[Javier Martinez Canillas <javier.martinez@collabora.co.uk>]

Hello Lee,

Thanks a lot for your feedback.

On 01/20/2015 08:50 AM, Lee Jones wrote:
>> @@ -59,9 +60,17 @@ struct cros_ec_command {
>>   *
>>   * @ec_name: name of EC device (e.g. 'chromeos-ec')
>>   * @phys_name: name of physical comms layer (e.g. 'i2c-4')
>> - * @dev: Device pointer
>> + * @dev: Device pointer for physical comms device
>> + * @vdev: Device pointer for virtual comms device
>> + * @cdev: Character device structure for virtual comms device
>>   * @was_wake_device: true if this device was set to wake the system from
>>   * sleep at the last suspend
>> + * @cmd_readmem: direct read of the EC memory-mapped region, if supported
>> + *     @offset is within EC_LPC_ADDR_MEMMAP region.
>> + *     @bytes: number of bytes to read. zero means "read a string" (including
>> + *     the trailing '\0'). At most only EC_MEMMAP_SIZE bytes can be read.
>> + *     Caller must ensure that the buffer is large enough for the result when
>> + *     reading a string.
>>   *
>>   * @priv: Private data
>>   * @irq: Interrupt to use
>> @@ -90,8 +99,12 @@ struct cros_ec_device {
>>  	const char *ec_name;
>>  	const char *phys_name;
>>  	struct device *dev;
>> +	struct device *vdev;
>> +	struct cdev cdev;
>>  	bool was_wake_device;
>>  	struct class *cros_class;
>> +	int (*cmd_readmem)(struct cros_ec_device *ec, unsigned int offset,
>> +			   unsigned int bytes, void *dest);
> 
> Is this safe?  Are you sure it's okay to provide an interface from
> userspace to read (kernel?) memory?
>

This interface is not to read any kernel memory but only the memory mapped
I/O region for the Low Pin Count (LPC) bus. So user-space only can choose
and offset and a number of bytes using the CROS_EC_DEV_IOCRDMEM ioctl cmd
which uses the following structure as argument:

/*
 * @offset: within EC_LPC_ADDR_MEMMAP region
 * @bytes: number of bytes to read. zero means "read a string" (including '\0')
 *         (at most only EC_MEMMAP_SIZE bytes can be read)
 * @buffer: where to store the result
 * ioctl returns the number of bytes read, negative on error
 */
struct cros_ec_readmem {
        uint32_t offset;
        uint32_t bytes;
        uint8_t buffer[EC_MEMMAP_SIZE];
};

The cros_ec_lpc_readmem() handler that the function pointer is set only
reads bytes from EC_LPC_ADDR_MEMMAP + offset if offset < EC_MEMMAP_SIZE
and the data is copied to the user-space buffer from the structure passed
as argument with copy_to_user().

So in that sense is similar to the spidev or i2c-dev interfaces that are
used to access these buses from user-space.

Best regards,
Javier