Question: I'm trying to write driver for rtl8139 for linux 2.6 from scratch. I've already written TX path, but I have some problems with RX. 

I put RX into promiscous mode and receiving RX irqs. I set RBSTART into physical address of allocated memory by kmalloc. 

I don't know how to find out how many received packets there are and how long they are. 
I thought that ERBCR, CAPR, CBR registers tell it, but they are == 0.

Maybe I'm doing something wrong? How to find out anything about received packets?

Answer: 
I answer to my question myself.

The received packets are located starting at RBSTART. The first two bytes of rx-ed packet are status bytes, and the next 2 are length of the frame + 4 bytes of crc.

Maybye someone find this info helpful.
On receiving a packet, the data received from the line is stored in the receive FIFO. When Early Receive Threshold is met, the data is moved from FIFO to Recieve Buffer. 

So, once you get an interrupt. You need to check the Interrupt Status Register for ROK. Then check the Early Rx status register which gives you the status of the packet received. If EROK is set, then check the Receive buffer status for ROK. Check for are any errors in the ISR and ERSR. Also check your Rx Configuration register for the threshold configuration for Rx FIFO, RX buf length.